There will always be RM routes for the 737 - STN-EDI, EXT - various.
The containerised ATR in its present form does not take the industry standard containers that the 737 and others use, though I believe it is viable to alter the a/c structure so that they can accommodate them - at cost!
I guess if you had a number of containerised ATR's using the special bins in one network it could work, but if they are feeding into a 737 network?
With Air Contractors, Farnair receiving ATR's and BAC looking at options, now could be a good time to get an ATR rating as opposed to the shed/F27. Might explain why Skylines courses are filling up so fast!