The only point to using an amnesty is to: get below the cap and/or free up space to sign other FAs. I don't think the latter makes any sense in Toronto. Now, if the NBA ends up with a hard cap in the $45mm range you may need to buyout someone to get under that total in order to field a roster. In that scenario, it's most likely a big salary like Calderon who gets the nod and I don't think it's by choice but by necessity.
Not sure paying Calderon ~$20mm just to turn around and pay $20mm for 2 average players makes any sense for the Raptors.