Uh, why would let's plays on youtube be a completely unbiased dataset? It is well enough as one measure, but by no means special sounding. Probably the equal of google images in terms of well-constructed studies (i.e. not one). Go ahead and count them up and add it to the list of lazy measures, though, if you want.
First of all, I made no claim as to the bias, or lack thereof, of such data. So please, don't attribute such a claim to me.
Second, it's not the number of let's plays, but rather the size of the audience, that makes up the pertinent data. It's very difficult (impossible, perhaps) to control for factors such as the skill of the 'caster, the popularity they carry over from other, unrelated videos, etc... however, the number of DF popularizers using IronHand and Phoebus, on one of (if not THE) most popular media for introduction to games, is overwhelmingly greater than those using plain ASCII. Considering that number of views is one of the primary reasons for uploading videos to YT in the first place, and that, even on a cursory read of the data, videos using graphics packs are orders of magnitude more successful in garnering those views, these data are relevant.
Perfect? Of course not. Relevant to this discussion? Absolutely.
A look at these data is analogous to the use of Nielson ratings to determine the popularity of a television show. It says nothing about the quality of the medium; however, it speaks volumes to the popularity of it. In this case, graphics packs are far, far more popular than plain ASCII.
The above is not an argument against the use of ASCII in screenshots, etc., nor an argument for the usage of tilesets. If, however, the goal is to be as relevant as possible, to the greatest number of players and potential players as possible, this is evidence that enforcing a standard of ASCII only would be, at best, counter-productive. At worst, insular and elitist. Considering that DF is entirely funded through donations, I find it hard to comprehend how anyone would wish to alienate any other would-be player - especially when there are plenty of popularizers who are quite willing to spread the word that DF exists and is entirely comprehensible (despite its reputation) - simply over a desire to maintain some sort of illusory 'purity' or the like.
One place where the enforcement of a standard makes sense has nothing to do with the graphical representation of the game: the use of the DFFD for uploads of saves, mods, and the like. But that, I suppose, is well beyond the mandate of this thread, so I'll leave it there.