I get your point. I do understand that Ayres having a contract makes him expendable. I would add he gave us solid minutes against the Lakers the previous playoffs, and he gave us solid minutes against OKC in the WCF these playoffs too.
IMO, it's not about wetting pants, he's a marginal talent after all. It's about the versatility of having a big man that matches up well against tall, physical centers. Tiago normally takes on that role, but when he's in foul trouble, that's Baynes. Neither Bonner or Ayers fit that bill. Bonner's role is completely different, he's a floor spacer, he's a natural backup to Boris. We're talking $2m for marginal talent that's a good fit for our roster. If it's more than $2m or more than one year, then sure, the equation changes.
It's not like the Spurs are tiptoeing the luxury tax or anything like that. I also think, in a vacuum, he would be more valuable than Ayres or Daye.