I can lots of benefits.
1) As Patchick stated above, Cutler's 2015 salary is only guaranteed for injury, so this gives them flexibility to cut Jay if no viable trade offers are available.
2) If Clausen somehow looks competent, Jay looks like a convenient fall guy allowing Trestman to keep his job.
3) There are rumors that Jay lost the confidence of the locker room, and/or that he's not following the playbook properly, so maybe Trestman is forced to do it or risk appearing too timid to be respected leader.
I can't really see how this hurts their chances of trading Jay. He's too well known in the league for a game or two at the end of the year to really sway anyone's opinion of him. It's not like he was going to light up Detroit playing behind that OL, either.
Frankly, I'm only surprised that it didn't happen sooner. No, check that, I'm also surprised that they signed him in the first place. I wouldn't have given Cutler Matt Cassel's contract, let alone the one he actually got.
All that said, I have to admit that Jay came off very impressive in his press conference yesterday. He was very supportive of Jimmy, admitted he didn't take the news well, refused to get into a pissing match with Trestman and took a good portion of the blame for his benching.