They've gained already. if you think that the PS3move was the original version before going MP, others will and that might lead to people deciding to go with the move version over the others because they will somehow think its better. i know it sounds silly but there are a few diehards who still think in that way and importantly have money to spend.
Plus as what the others sensibly said, Sony got free press for several months, so they have the advantage of getting it out there that they had it "first" to gamers.
Makes you wonder what exactly does the Ps3 have as an exclusive if they're getting any. On my count Sega has given Kinectic about two/three so far.