During the rebound ace era, the Kooyong Classic was seen by players such as Federer, Agassi, Roddick, Nalbandian and Safin as the best preparation for the Australian Open, instead of playing in one of the lead-up ATP tournaments. The guarantee of 3 matches against quality opposition and being able to set up base in Melbourne for a bit longer must have been attractive to them.
Hewitt always felt he should be loyal to the smaller Aussie tournaments Adelaide and Sydney (plus Adelaide was his home city tournament where he made his memorable 1998 breakthrough) and would decline invititations to play at Kooyong every year until 2011.
However after the Aussie Open changed surface to plexicushion in 2008, and the money spinning Abu Dhabi exho was set up in 2009, the Kooyong exho has suffered big-time. Nadal playing there next year would give it a huge boost.