It seems like all high profile players play/used to play it.
Federer won it with Martina Hingis
Nole already confirmed his participation (with Ivanovic) for next year
Murray is also playing (with Laura Robson)
Hewitt is playing as always.
So why does Nadal always miss it? IMO it's a fun event where you don't need to try to play your best and it would be HUGE for the tournament if Rafa (reigning US Open champ
) participated instead of killing himself in masters events in Doha and such.