I am guessing that the reason that the TBM and B5N do not have formations is because that either their usage was not as a formation bomber (more for the B5N I imagine,) or because gameplay wise carrier launches to not really work for formations, whether mechanically or visually.
I was thinking that it could be a gameplay issue (non issue, if the total amount of ordinance allowed from a CV launch would be less).

However, I am in favor of seeing this for the sake of increasing the fleet impact on attacks.
I could be wrong, but it seems that LVTs spawn further from shore, and faster and better capable bombers have been added that sink CVs in one pass. There is something to be said for adding some offense to the fleet (there again this could be balanced by limiting ordinance).