Because Keim has shown his hand--there's not going to be a top QBOF drafted this year. If they were thinking of going that route, we wouldn't have gotten a 'top backup'--the rook would have been the backup, learning as much as possible. The 3rd QB is a scout/practice squad player.
(a) If the Cardinals don't get a viable QBOF in the draft, you can complain about that then. I don't believe that signing Glennon has any bearing on that issue.
(b) As others have pointed out, signing Glennon doesn't automatically make him 2nd string and a rookie 3rd string. If the Cards want to develop a rookie, they can make the rookie 2nd string and Glennon 3rd in practice - with Glennon being the first to replace Bradford if necessary before the rookie is ready to go, and the rookie remaining 2nd string behind Glennon until he's ready.
If the Cardinals draft a late round rookie and put him at 3rd string, you can re-post everything you've said then.
...dbs