Some intelligent responses by my readers have got me churning over the particularly complicated issue of the PIE subjunctive. Thank you very much. In the exchange, I'd just realized a special issue that I'm still fuzzy about in the standard theory - what the form of the subjunctive really should be in Jasanoff's model.
The subjunctive in Proto-Indo-European (PIE), which denotes hypothetical actions and states, is assumed by quite a many Indo-Europeanist to be marked with primary endings. The "primary endings" are also commonly believed to be distinguished from "secondary endings" by means of a "present tense marker" *-i. Now, my mind instinctively refuses to process all of these claims at the same time because of implicit paradoxes in this account. When trying to understand the PIE grammatical structure as a whole, the many traditional tenets of Indo-European linguistics such as we see in the common interpretation of the subjunctive mood makes PIE seem to me like a language spoken by extraterrestrials rather than a genuine human language that follows human tendencies of speech and comprehensible notions of basic semantics. I have a hunch that one of you out there have felt the same way but couldn't quite put your fingers on it.
So let's ponder on this subjunctive for a moment. This in-depth site from the University of Texas on Indo-European syntax explicitly supports my paranoia by recognizing that "Indo-Europeanists have wondered why the optative has secondary endings, inasmuch as primary endings came to predominate in the subjunctive." Yes, why? Why on earth would endings used in present-futures be associated with the semantics of a subjunctive yet absent in the optative if both the subjunctive and optative convey future reference through the lense of potentiality and desire? The author here claims that it is because the optative is older than the subjunctive, but because I have to seriously consider Jasanoff's view that the subjunctive is the source for thematic indicatives, begging necessity for the subjunctive in the earliest stages of PIE, I can't swallow some of the author's well-researched claims here, even though in all honesty it's a very plausible answer.
Part of the problem here is that all sorts of terminology in this field that we take for granted are entirely misleading for even a seasoned scholar. Don't just question what you read but question the true meanings of the very words that linguists use. IEists for example volley terms about like "aorist" (aspectual or tensal?) and "markedness" (phonetic or inflectional?) within a variety of sometimes contradictory contexts and it's important to recognize the shades of subtlety.
The true meaning of the marker *-i of the primary pronominal endings that both the indicative and the subjunctive supposedly don is a tantalizing universe that deserves a blog entry of its own but for now, let's just accept that it does not mark tense as is often mistaken. This is because of things like the negational mood with examples like *ne h₁est which are able to refer to a non-preterite meaning of "it is not" or "it will not" just as easily as preterite "it was not". Why should a present marker be absent in negative statements? So we must exorcise from our brains the outdated notion that this marker is actually a tense marker at all, despite still being called such, since it is absent in so many places where the present-future is conveyed. Rather it is, with strong reasons built on structural linguistics, a marker of mood (as argued on the above cited website and renamed a "declarative marker").
Adding to this however, I would argue that *-i was in fact a specialized indicative marker, restricted solely to the affirmative, present-future subset of this indicative mood. A kind of "imminentive" or "evidential" marker attached only to real actions, never hypothetical ones, whose ongoing or future occurence is factual with absolute certainty from the point of view of the speaker. Giving it the meaning of evidentiality works exceptionally well with its proposed etymology from an originally independent temporal-spatial "hic-et-nunc" particle attached to pronominal endings in pre-IE, in turn fashioned from the demonstrative stem *i- "he, she, it".
This of course means that the original subjunctive would have to be, like the optative, marked with secondary endings and that later IE languages have confused the original state of affairs because of a close relationship between the uncertain subjunctive ("would do") and the certain future indicative ("will do") . This reasoning is further supported by the fact that the subjunctive so easily vanished with little trace from Anatolian (aside from the handful of thematic indicatives that Jasanoff claims derive from this subjunctive). This grammatical disappearance would be easy to accomplish had the subjunctive been an inflectionally impoverished category in comparison to the indicative and lacking primary/secondary contrasts. Not so easy however, if it had been like the complex multidimensional tables of inflections that later Greek and Sanskrit give us.
A final noteworthy confusion stems from what the "present subjunctive" (*bhérēti) really is. It is certainly called a subjunctive because of the thematic vowel added to the stem as we see it in Classical Greek or Sanskrit, but in reality it's usages and forms were often hard to disambiguate from the "future indicative". In these languages we happen to see a lot more complexity than what must have originally been, such as "present" subjunctives versus "aorist" subjunctives.
So, I should think that the real subjunctive in PIE would be the form using the secondary endings (*bhérēt), a form afterall that is then sufficiently distinct from the athematic (eg: *bhērti) and subjunctive-derived indicatives (eg: *bhéreti) to allow the maintenance of these subtle contrasts for the long haul as PIE fragmented into the various branches we now see. If the subjunctive were so similar to the present indicative because of its use of primary endings, is it really likely that it could be maintained so strongly as a seperate category for so long to have still appeared in Sanskrit and Greek unscathed? Personally, my brain just can't process such an unlikelihood.
But this was just an idle thought. Do with it as you please.
 (05/25/07) Actually upon deeper reflection while sitting at a coffeshop in solitude today, it might be better to read "continuousness" (an aspect) rather than "present-future" (a tense) here. This would more accurately describe the state of affairs of the earliest layer of IE, that is, the one that includes Anatolian. Afterall, if thematic verbs are more recent, the aorist and past blur together. Ergo, present-future is a later innovation and the earliest layer is purely about aspect, not tense. Then I realized that this *-i marker has striking parallels in both usage and etymology to Mandarin zai which is also used to mark the continuous (eg. 我看 wo kan "I look"; 我在看 wo zai kan "I am looking") and comes from the locative verb "to be at" just as IE's marker comes from a locative demonstrative. Nifty!