Leach gives your offense versatility. Caldwell just needs to be more creative with his I-formation scheme than Cameron was. When you get a lead in the fourth quarter, you still need to be able to crunch some clock sometimes. Leach helps you do that. There are still going to be teams that you will need to get physical with as well.
Leach is a guy you keep if your able to but sacrifice if you must. If letting go of Leach allows you to bring in a higher quality LT, DT then you have to do it IMO. If letting him go will not allow you to get another impact type guy at a position of need then you keep him.
Vonta is getting the 1.3 signing bonus money regardless of whether he plays or not, so that's not a factor. The 3m salary is the issue and I think something can be done there. Restructure/extend him at a bargain or let him go. They used him as a receiver a few times out of the backfield in an effort to use him outside of a blocker. Normally the defense avoids his hits so the idea of a DB running at him to initiate contact is my idea of fun.
Not a must keep, but he allows the offense to dictate to the defense when he is on the field. Seeing leach in the backfield helps with the play action fake. Drawing the defense up in anticipation of a run will most likely get one one coverage for the receivers. If he stays cheap, keep him. You know fans love a hard hitting fullback like Leach. Foster missed him these last two years.
You don't want to fall into the trap of being a finesse offense that lacks physicality when it is needed. The Ravens just beat up on a team like that in the AFC Championship Game.
I don't see any team that would value Leach more than us. I think he restructures.
Also, the Jets would probably be all over him, they'll have to be able to run the ball if they want another year with Sanchez as the starting QB. They'll need to be able to run with any other option anyway, Ryan Nassib, Geno Smith, or Matt Barkley, aren't day 1 refined passers like an RG3 or an Andrew Luck.
Our offense doesn't become finesse is Leach leaves. Especially considering he played less than 50% of the snaps this past season. Leach definitely adds a certain physicality, but I don't see our offense being finesse when we have guys like KO, Yanda, Oher and Pierce on offense.