The thing is that language skills have their specific uses. They are not exactly suited to every character in every situation, like a weapon skill... BUT there are situations where they are prime.
I'm saying this because the main inspiration for this was an event that happened today. I was playing "pure mage", was in the Mantellan Crux, and my weapon was sheathed - so there was I killing everything on the way, when I jumped into the pyramid pit and right in front of a Daedra Seducer. And to my surprise, the Seducer just stood there. Thing is that I never had to "talk" to her in dialogue mode... And the only thing of note was that my weapon was sheathed. And I had about 4% Daedric skill, for the record because I never bothered with it. (FYI, she was actually looking at me, so it wasn't a function of stealth).
Now the reason why the canned "Mage", "Healer" classes and others have these skills is probably vastly underestimated. Not every class is conducive towards pure hack and slash, not every class is conducive towards stealth or even illusion - the Healer is a case in point. And when you're essentially a wimpy guy with little combat skills, no armour skills and just short blade on the minor slot, it might be best to evade the fight whenever you can; a character who uses the Mages Guild Trainer to boost his Daedric skill up there has a potential chance of going all the way through Daedric lairs and the Mantella without fighting a single Daedra, provided that he maintains a cautious attitude - it's easy for spellcasters, since they don't have to carry a readied weapon at all.
... and contrarily to what I thought, there's no need for any special dialogue clicking. So long as the critter is not turning its back on you, but actively glancing you without making a move, it means the language skill comes in handy. It is automatically checked.
This just gave me a bright idea, alas. Instead of the traditional hack and slash, I'm going to make the exact antithesis of it - more of a stealthy character savvy in one or two languages. And for those who think that is quite hard to level up Daedric when you're low level, well, Trainers are there for a reason. In fact Trainers tend to be quite underestimated sometimes. Just loot a graveyard or go dungeon crawling, and burn all that extra gold you're not going to use with Training.
Now tell me what other TES game allows you this?