I don't understand the line of thinking here. I could go on and on about why signing him would be a smart move, examples: young, cannon arm, potential, etc. But the thing that it boils down too, the thing that is ALWAYS the deciding factor: MONEY.
Hoyer is on the books for $2M dollars this season. If the Cardinals can sign JaMarcus for league minimum, then not only does Hoyers $2M come off the books (cut or trade), but JaMarcus salary wouldn't count against the CAP because he wouldn't have a salary that is top 51. That in turn gives the Cardinals $2M more dollars in CAP room. I don't see a negative thing here other than the possibility that he fails. But how many 3rd string QB's really have a high chance of success?