Usually aftertouch isn't polyphonic and most synths provide channel aftertouch where whichever key has the greatest pressure is applied to ALL notes on that channel.

But even channel aftertouch can be very effective. Uncle Dave, if you haven't found aftertouch to be useful you simply haven't tried it with the right patches. Obviously it only applies to sounds that are sustained while the pressure is applied such as pads, electric pianos, brass parts etc. I particularly like aftertouch to increase/decrease filter & resonance on pads and also to provide tremelo or vibrato on electric piano sounds. It's great to hold an electric piano chord and bring in a tremolo effect as it rings off.