The flaw with that is shown when you actually look at the reserve team lineups last year. They were constantly filled with trialists and academy players, which really isn't ideal. I think sending a few of these guys to actually fight for minutes in a professional environment will be beneficial in the long term, rather than just rotting on the end of the bench and playing in a handful of glorified scrimmages. Also, USL isn't the highest standard, but some of the better teams are definitely better, and certainly more cohesive, than MLS reserves.
I have heard that the longer term plan is to integrate MLS reserve teams into the USL league structure, perhaps even as soon as 2014. It was obviously a little too late for both parties to get that sorted out this season.