Collison is not a "proven starter" He's actually never stuck anywhere as a "starter" since he's been in the league. He's a good player, but there's been issues with his lack of consistent play throughout his career. Dude got bounced for Mike James for pete's sake.
Where Collison excells is he has value at both the 1 and 2 guard spot, he's a good consistent 3pt shooter, and he can D up a bit.. He played a ton with Chris Paul last season and he was in the Clippers closing line-up at the 2 guard when they went Cp3-Collison-Crawford-BG-Jordan. 23% of his minutes last year were at the SG spot. I think he makes a ton of sense there next season too in which we don't have to rely on Stauskas or Ben to close out games.
Fact is, our team gets a lot worse if IT isn't resigned, especially at the $5-$7mil valuation he's going to get. Collison-Mac-Ben-Stauskas is a disaster heading into the season. It lacks consistent ball-handling, playmaking and non-dependable scoring. IT-Collison-Stauskas-Ben-Ray turns the backcourt from a huge weakness into something that looks like an NBA backcourt. In theory, there's scoring, defense, playmaking, and shooting throughout that line-up. We'll never be stuck playing Outlaw at the back-up 2 guard, nor be stuck playing Ben 30+MPG.