I'm not too thrilled about some of the compositions comprising of more than one of the same class, but then again, it should be expected for the smaller amount of available classes. I think that if the 8th (?) class is a Mesmer, then it will definitely provide some depth into though comp's.
Keep in mind that those are just one blokes opinions on the comps and what they should comprise of, so I'm sure there will be many different set-ups come release
PvP will still be absolutely amazing no matter what though
Well, if you look at all the comps all 7 classes are represented in one form or another. They all excel at different roles, but all are useful.
As far as using more than one of a class, in any pvp game you always hedge your bets on what works best for your team. In a game like WoW people like diversity because of buffs. With pretty much all the skills having some kind of secondary effect in GW2 you've got a lot of utility without having to necessarily use different classes.
The design of bringing people, and not classes, fits perfectly for PvP. You could, theoretically, stack 5 of one class and still be competitive if your team is good. Variety will always be attractive because it'll be harder to predict and counter, but they really want to put the skill of the player at the forefront, and not their abilities.