I think Sony could see that there might be some gain for them in this. But I think to lure a substantial number of 360 owners to PS3 for vs XIII you'd need to see FFXIII achieve multi-million sales on the 360. Otherwise the luring effect will be pretty small.
Ask some of the 360 only FFXIII guys here whether they'll be picking up a PS3 for vs XIII? You'll probably mostly get "no" and the rest are probably maybe. I doubt there would be many yes's.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix