There is no problem at Frankfurt just going from Arrivals to Departures without going through immigration - and I'm sure you could check in at the gate.

However, immigration is very efficient there, and if you're on time, I'd go through just to get another couple of stamps in the passport (One in, one out). Also, if you wish to get to an airline lounge, you may well need to go through immigration anyway (depending on which terminal you're in). FRA's a pretty good airport and there's a reasonable selection of shops on both sides of the line.

Conrad