The biggest thing Davis brought to the table in the preseason was being able to evade the rush.  I wasnt blown away by his arm (didnt think it was bad either).  Having a mobile backup makes sense in that the first reaction of most coordinators when a backup comes in is to blitz.   But, I think resigning Clemons is a good move.  A backup QB, particulary a third, should be; inexpensive, a good team guy, know the playbook, be experienced enough to help prepare the D for the following week.  Checks.  I think Davis would be used to help prepare the D for Kap and Wilson however.  Dont see Clemons running a lot in practice.  
One thing for sure, we know what Clemons brings, we dont know in total what Davis can be.  So, give Davis the reps a #2 gets and see if he develops.  Nothing to lose.