Hi Bluezplayer
You are right that adding more ROM does not translate into more expressive instruments, but it does add to the realism of percussive and plucked instruments. ROM is used in sample-playback synthesis, which is weak when it comes to real-time control to emulate wind instruments, for example. (I should add that Scott Yee, who is an order of magnitude better musician than I am, is pleased with the way his PSR-2000 has dynamics built into the samples themselves.)
Analog synthesis and analog-modeling synthesis are very expressive technologies. Physical modeling is good for emulating expressive acoustic instruments. These can be added to the Motif and to the 9000Pro as plug in cards.
However, even if you have the synthesis capabilities, you need a way to control them. On almost all arrangers, the emphasis is on buttons rather than on continuous controllers. For the most part, arrangers are limited to a pitch bend wheel, a mod wheel, aftertouch, and footpedals. Aftertouch is absent or poorly implemented on most (but not all) arrangers. Someone pointed out under another topic that no arranger has a ribbon controller. Compare the continuous controllers available on arrangers with those available for analog synths or the new Neuron.
I can't blame the manufacturers too much. I remember someone who bought a PSR8000 asking how to turn off the aftertouch. But as arranger keyboards become more mainstream (in America, they already are in Europe) there will be more demand for realism and expressiveness. Hopefully, the response from manufacturers will not be long in coming.