I think Ian Book is considered an emergency QB, BUT if Siemian is injured, he would probably be the candidate to step in at QB and finish the game. This has less to do with "beating out" Taysom Hill than it does with gameplan.
If a Rookie QB gets the call over a vet because the vet takes snaps at TE during the week then I think the Rookie is the better option as the vet should be able step in and run the plays he knows since he's been here 4 years