Great way of supporting that with arguments.
The fact that with AS Roma it did not work out doesn't mean he is a very good goalkeeper. Just look at vdSar, who totally failed in Italy and after a period at Fulham really showed what he is capable of at Manchester United.
1) Stekelenburg has good reflexes
2) He is capable of playing the role we as Barcelona want from our goalkeeper; sweeper keeper. Being able to play as last man and help the build up.
3) The sheer fact Roma do not want him as #1 goalkeeper makes him CHEAP. Let's finally NOT pay tens of millions of euros to get players to Camp Nou... (Song, Alexis, Cesc, Zlatan, Villa and so on!)
I am not saying he is the absolute best keeper out there, but I am saying that he is up for grabs now and does possess the characteristics of a Barcelona goalkeeper. And that is something 'better' goalkeepers can't easily say.