Senna has not had enough of a chance to prove himself in F1. After all, he was GP2 runner-up in 2008, so he has talent.
2010: HRT. Don't need to say any more on that year.
2011: half season watching, second half in crap Renault. Faced up Petrov quite well, considering Petrov had 2 years to acclimatise with the car, and Senna had only driven it on 2 occasions before Spa FP1 (pre-season and Hungary FP1).
Everyone rates him down because they think his only saving grace is his name, but they forget his history in lower formulae and his unfortunate circumstances in F1. Add that to Viviane stopping his career after Ayrton's death, when he should have been developing his skills, and you have a pretty unlucky driver. Basically, I don't think he's as good as he could have been, but better than people seem to think.
I say: Give him a chance. A proper one.
Williams may be the way to go, as they know exactly what was wrong with the FW33 and didn't bother to fix it for 2011, concentrating instead on 2012 as they were never going to beat STR. They have Mike Coughlan so they'll steal some designs from some other team (sorry, I had to), and they can fix the problems with the FW33, and a Renault engine, meaning that the FW34 has potential. Get rid of Maldonado as well and his dodgy money. If it is found to be illegal then he has nothing to offer other teams without money. Senna will probably be able to get more than Maldonado anyway (and probably already has).
If not Williams, then I don't know. Another year on the sidelines wasting away is just no good, and there's no point sticking him in a Caterham as it will be at the back of the field.
If Perez leaves Sauber for Ferrari (assuming they dump Massa), then Sauber will have a choice of Massa or Senna. I'm inclined to think they will pick Senna - he has money, potential, and Massa is nowhere near as good as he was in 2008. Plus, Sauber has developed a reputation for being great at developing rookies (remember Raikkonen?), so this would be the perfect place for him to settle down until a larger team snatches him (McLaren if Hamilton gets fed up?).
He's 28 now, if he's going to get anywhere in F1 he needs to get a move on, and refuse to settle for pointless 3rd driver roles.
End of rant.