I believe you're more likely to need (demand) fully weighted keys if you have a long history (background & training) playing the acoustic piano. The weighted action allows you to play the electronic keyboard utilizing traditional acoustic piano playing techniques. This includes achieving better pianistic control: highlighting inner voicings, dynamics, and other uniquely pianistic features. On the other hand, when playing other instrument sounds (voices), I always prefer the light weight synth action.
Btw: It's important to note that BOTH the Yamaha 9000pro and Ketron SD1 do NOT include fully weighted keys so don't respond the way a true FULLY weighted piano action synth or acoustic piano does.
Coming from a lifelong background of playing acoustic piano, it was a difficult challenge to adapt to a synth action kb. Still, because I was no longer willing to deal with hauling 75 lbs around on my back, I relented and spent 6 months at NASA
to adapt to the feeling of space age weightless-ness.
I'm now GLAD I did. Admittedly, there are still a few pianistic devices not able to be achieved on my synth kb, but overall, I'm very satifisied, the biggest satisfaction being the 50 lb difference.
It may be interesting to note that over the years of having to switch back and forth between the fully weighted keys on my Steinway B Grand and the weightless feel of the Yamaha PSR2000 and even the Tyros, that I'm now able to comfortablly go back & forth between the kbs without difficulty. I"ve just had to learn to approach playing each of them differently.
Scott