As depth, I wouldn't mind him. Milan could use a backup for Pirlo.
Here's the thing for me. All about price. I'm not a montolivo fan. I actually get into lots of arguments with people who claim he's 'better than pirlo' (he's in no way). But he's HARDLY a bad player....
Did you know he was actually top 10 in serie A (I believe it was 6th) in tackles won last year? He's quicker and bigger than Pirlo. Not near his technical level but more of a pure CM. Tactically he's pretty intelligent too, intercepts a lot of balls by reading passes
I've liked him a lot less since he got put in a xabi alonso role. But like Pirlo, he does the job he's asked. People act like he or pirlo are static. They aren't. Depends what role they play. Does Pirlo ever look static for Italy? No it's because for Milan he was always asked to sit back and orchestrate.
I remember when Fiorentina used a 4-3-1-2 and he was a more active side CM with technical qualities.
I feel if the price was right, Allegri could find good use to him.