I can't remember the first time I was invited on Teamspeak (some people use Ventrilo as well-- I've got both free programs installed on my PC, just in case) for an instance-- for some reason, I'm thinking it wasn't until I got invited to a raid at level 60, but considering the way that things are now, I'm really surprised it wasn't earlier. Maybe it's just because almost every instance I run is with guildies, but TS is basically a requirement for grouping-- a requirement that most of us are happy to comply with, but a requirement nonetheless.
Yakov isn't so compliant-- he says Vent is a crutch, and the fact that every group he enters asks him to "jump on Vent" is rubbing him the wrong way. He says a simple 5 man run doesn't call for using Vent, and that he'd rather listen to his music then his guildies chatting it up on Teamspeak.
I disagree-- not only is it more fun to chat with people on Vent (I tend to know the guildies I've talked to on TS better than the others), but it's just plain helpful, whether the instance is hard or not. If you're disappointed that your guild isn't asking you to come on more instance runs with them, and you haven't downloaded Vent or TS yet, that's probably why.
Of course, like all things, Vent can go horribly wrong. You can be annoyed by your guildies' voices (I'll just say that some people's accents are a little offputting and leave it at that), or like Yakov, you can rather listen to music then random guild gossip. But especially if you're a player just getting to the endgame where there's a lot to learn, jumping on Vent is one of the best things you can do to make sure you know what's going on when. It's not a difficulty thing, although coordinating a pull on TS is much easier than typing strat in the chat channel. It's more of a communication thing-- the more and the faster comm there is, the better you'll all be.