If Glennon or McCarron were considered NFL starters, or in demand, they would have been dealt by now. You really think a team wouldn't give up a 2nd or 3rd round choice for a guy they thought could be their QBOF? And, if any team offered a 2nd round pick for Glennon or McCarron, their bags would have been packed a long time ago and they'd be on another squad today.
Look at all the junk that was out there at the QB position, hell Matt Barkley started. Glennon and McCarron are considered in that same group right now. SF went with Gabbert and Kaep rather than facilitate a trade. The Jets stuck with Fitzpatrick and Geno. The Browns chose to pursue RG rather than those two.
They are limited QB's who may fill in nicely for a short period but once teams get film on them and gameplan for their deficiencies, they'll likely struggle. See almost every QB in that scenario who moved to another team and struggled mightily in Scott Mitchell, Osweiler, the GB kid who went to Seattle, and so on and so on.
Conversely, if a guy can play, teams know it and will pay the price. That's why GB was willing to give up a 1st rounder for Favre even though threw all of four passes in Atlanta.