There are other advantages to having Belt start the year in Fresno that have way less to do with Belt's abilities and far more to do with other players. The team can only break spring training with 25 men on the roster. With 12 pitchers on the roster for sure, do the math: only 13 spots remain. Cody Ross and B Weezy are on the DL, so they don't count (for now). Sending Belt to Fno allows Nate and Ishi to remain, neither as talented as Belt but who bring other elements that have value, including Nate's arm in RF and Ishi's ability to come off the bench and draw a walk (see: Giants 2010 postseason). Aaron Rowand's contract is hanging around his neck and that of the Giants like a 7-ton boulder, and the irony is that both he and management would be so much happier if they no longer had to greet one another every morning. So maybe, by June, Sabean The Dealer can manage to work a trade/sell/barter with another team that needs a veteran centerfielder and has enough pop in its lineup already that Rowand's limp bat won't be such a liability. I have no idea which team that might be.
My roundabout point in all of this is that while Belt in the lineup is a benefit to the Giants, NOT having him in the lineup isn't a liability—the team isn't worse off without him. So keep him working in Fresno, give him more time seeing pitches, all the while allowing Nate, Ishikawa, and yes, even Rowand, to remain on the 25-man roster. Come June, let's see what's what.