From a purely selfish perspective, the principal benefit benefit here is the ability to earn UA miles (and I expect PQP/PQF) on B6 MCO-Caribbean flights, which would ensure that we give them our business over other carriers on the couple of trips a year we do on those routes. I imagine capturing the UA travelers flying those routes who are currently shopping among B6/WN/AA/various other carriers is enough of a benefit to B6 to make them willing to swap JFK slots for EWR slots.

There was a long discussion here a few months ago about UA and a FL hub and it was clear from that thread that there is a solid population of FL-based UA flyers currently shopping around for Caribbean flights since UA is completely impractical on those routes.