Just because a vendor doesn't happen to have the skills you want, doesn't mean the skills aren't available in game. You just don't get the skill exactly when you want it. It adds some variety to the game even if you were to do a new run with exactly the same char build.
I think it adds to the enjoyment of the game. And I don't think that is absurd.
The issue is that you only get a handful of skills per level in that skill, which mean that, at least for early game, you can quickly fall into 'this character is unplayable because he doesn't have the only offensive ability he could have'.
It's less of an issue for higher-level skills as you would have already a good deal of versatility, but at first it can cause problems.
This is why, for me, a good balance would be that skill tomes of low level stuff can be gotten quasi-reliably so that your character gets playable, but for higher level stuff it's much harder. You can 'expect' to have them by endgame (especially if some are handplaced in some dungeons), but you could miss some spells you would want for a good long time (or even, if unlucky, not get a couple).
