In a 10-man raid, you have six DPS slots (or five, depending on how many healers you're running with). Three specs can provide Replenishment, out of 21 DPS specs (I'm assuming Feral druids, and all death knights, are equally likely to be DPS as tanks). If DPS players are randomly selected, then, the odds of zero out of six DPS providing Replenishment are (18/21)^6 = 39.7%. In other words, you're only 60% likely to wind up with Replenishment in your 10-man.
But maybe Ghostcrawler was just talking about 25-man raids. What do the odds look like there? Let's say you have 3 tanks, 7 healers, and 15 DPS. The same 21 DPS specs exist, and the same 18 of them don't give Replenishment. The chance of not having Replenishment with 15 DPSers is then (18/21)^15 = 9.9%, so you're about 90% likely to have Replenishment in a randomly assorted 25-man. This number is high enough that it makes sense to start talking about assuming a raid has Replenishment, and of course it's always possible to purposely select a shadow priest, ret pally, or survival hunter.
Still, it does partially do away with the notion of "bring the player, not the class." Consider that each class has three specs, and there are three specs that have Replenishment. Numbers wise, then, saying "Replenishment is mandatory" is not really any better than saying "rogues are mandatory." If there was some mandatory rogue function, all 3 rogue specs would provide it; all 3 Replenishment specs provide Replenishment. When you're putting your raid together, there's a similar constraint: have one DPS slot left, and no Replenishment yet? It's going to the Replenisher, just like it would have to go to the rogue if rogues were mandatory.
Aside from Razuvius in heroic Naxx (where priests are required, for mind control purposes), I don't think there's any other part of the current raiding game where it is assumed that you will have at least one of three particular specs in your raid. Debuff types (poison, disease, magic, curse) are typically dispellable by two classes, and any spec of those classes can do the job (with the exception of feral druids). I think upping Replenishment to being provided by six specs would be a big step towards making something that's supposed to be mandatory easier to access.
There's no particular reason it has to be a DPS class, either. Healing classes are the ones that worry most about running OOM; maybe Blizz doesn't want to make it harder for them to do that, but I'd certainly appreciate Replenishment on my priest. Paladin tanks also use mana. Let's imagine Replenishment is given to three additional specs, healers and/or DPS. There are 26 non-tank specs (again counting Ferals, and each DK tree, as 0.5). The odds then work out to (20/26)^8 = 12.3% likely not to have Replenishment in a 10-man (with 2 tanks), and (20/26)^22 = 0.3% likely not to have it in a 25-man with 3 tanks, a small enough number that you can pretty much write it off completely.
Of course, this analysis goes out the window somewhat when dual-spec comes along; if every hunter, priest, and paladin sets their Replenishment spec as one of their two specs, your raid is again almost guaranteed to have access to the buff. But then they have to gear for the Replenishment spec, and that may not be the spec they wanted for PvP/soloing, and so forth. It's probably to soon to predict what all the social ramifications of dual-spec will be. For now, if they're going to assume every raid has Replenishment, they need to give it to more players.