In terms of how it benefits the Grizzlies? Well, Belt is their best hitter obviously, and the Grizzlies and Giants organization need him to play everyday. That being said, while it would be good to get him some experience in the outfield to make him more versatile when he eventually gets called up again this year (he's not staying in Fresno for long, folks), he still is the Giants' first baseman of the future. Furthermore, while Ishikawa may not have much upside in terms of contributing to the Major League roster (I think it's safe to say he's probably a career Major League backup at this point), he should be in the Grizzlies lineup everyday, and mixing him and Belt up between first base and the outfield is the best way to give both of these offensive threats at-bats.
As for Pill, well, if you have to choose between Belt, Ishikawa and Pill for the first base position, Pill is probably the odd man out. He just doesn't have the power upside that Belt and Ishikawa have. However, OGC made a great point about Pill moving to second base on a perhaps more permanent basis:
"...If Pill can be converted to also play 2B, that would increase his value on the bench, because his hitting is OK for a 2B, not so much for a guy who can only play 1B. He could be like an Uggla where you give up defense to get the offense at best, though more likely a nice bat off the bench who can play multiple positions."
I like that Uggla comparison. Not because I expect Pill to be the next Dan Uggla (Uggla showed more power, and Uggla struck out a heck of a lot more in the minors; he had a 20 percent strikeout rate in his five seasons in the minors), but because I think Pill's offense is enticing when you look at it his numbers through a second-baseman's lens. As a first baseman? Pill probably projects to have Daric Barton power without the Daric Barton plate patience. As a second baseman though? His ability to make contact and hit 10-16 home runs and rack in a lot of doubles, would make him an intriguing middle-infield option, not to mention one of the better offensive middle infielder prospects in the Giants system at this moment along with Charlie Culberson and Brandon Crawford. (I'm just talking currently though, for there are infield prospects in the Giants system who obviously have higher ceilings than Pill; he's just one of the few that's closest to being Major League-ready.)
Of course, his offense will have to make up for what he'll give up defensively. In eight games so far this year, he has been decent in terms of making the routine plays (only one error in 32 chances), but he hasn't shown an incredible range (3.88 RF/G). This may be him getting just used to the position, but at six-foot, four-inches and 201 pounds, one has to wonder if Pill is athletic enough to be an average second baseman defensively.
This sudden position change makes Pill's prospect status a little more interesting, but I'm still keeping him at No. 22. He could rise if he continues his offensive production (specifically the power numbers) and manages to be decent defensively. With the Giants experiencing a ton of problems in the infield with Miguel Tejada's ineffectiveness and Pablo Sandoval's injury, Pill might be an enticing option to Giants management if he can develop defensively at the second base position.
The interesting thing though is how Pill's status at second base will be affected with the return of Emmanuel Burriss and Ryan Rohlinger to Fresno (both utility infielders). While the Pill experiment is an interesting development to pay attention to (simply because you want to maximize his value as a player and prospect), it may end sooner than expected, simply because Burriss and Rohlinger are players who are expected to get at-bats with the Grizzlies. I'm curious to see what direction manager Steve Decker will take with Pill.
In my opinion, the Giants organization (Not to mention Pill's professional career) would probably benefit more from this experiment at second base than plugging Pill back at first. We know what to expect from Rohlinger and Burriss in the middle infield and in the lineup. You can't say the same thing with Pill. It could be a bust, sure. But then again, there could be some surprising upside with Pill's position switch, and that certainly wouldn't hurt the Giants in the near future (especially considering Pill is on the 40-man roster).