Because if Neal is getting some of the backup SG minutes, then that means Ginobili and Green will be splitting the backup SF minutes. Remember, Leonard's minutes are going to soar next season and Kirilenko is too good to play something like 20-25 mpg backing up both forward spots. What they could (and probably would) do is start him at PF and bring Splitter off the bench. Of course, he'd have to be on board with becoming primarily a PF.
But the only way they could even make a legitimate offer at at this point is if they amnesty Bonner. Presuming they re-sign Ginobili and Neal for $8-9M and Splitter's starting salary is $9M (which it may not be, but either way, it's not going to be drastically lower), they could offer him approximately $7.5-8.5M.