They are and have been for the past two years ever since Guardiola handed Messi the number 10 shirt. He single handedly won Barca the league between March and May, absolutely scintellating stuff. Not particularly a one man team, but for the past 27 months it's been Lionel Messi and 10 supporting cast members.
Xavi and Iniesta have proven in the past for Barcelona they're not quite so good without Messi in the side whereas Messi's the opposite. No one pulling the strings in Barca midfield? He goes and does it himself. It'll take a 6 month injury to Leo for people to appreciate just how great he is though. He's not just any old special player, he's even better than Diego Maradona was at 23 years of age...
Rubbish, they've never been a one man team, and the idea that it's just Messi +10 is ridiculous, you couldn't just take out Xavi/Iniesta/Alves and throw in some mediocre players and achieve anything close to what Barca have done. Messi isn't the only player who's played a significant role, he's just easily the most impressive.
Xavi and Iniesta don't start playing worse when Messi is out, things just naturally look different (and less impressive) because Xavi interacting with Pedro (or anyone else, realy) is never going to be as impressive as Xavi interacting with Messi. That's not to say that they lose even the slightest bit in terms of their intelligence/creativity/passing ability etc etc. It's Barca as a whole that isn't performing as well when Messi is out.
Plenty of their best games have come during Euro 2008 and WC 2010, when they obviously weren't playing with Messi. Of course having Messi on the field makes things easier for them, but it doesn't magically change what they're capable of.