I don't use Palm personally, but if someone is using it, and if it is working for their system, then there is really no reason to change.
No system is forever, and there may come a day when Palm is no longer supported, or no longer works for them. In my opinion, that will be the time to change systems. Until then, changing for the sake of change is wasted effort.
I use PocketPC, and it is probably not the best platform from a technology point of view. However, I've been using it long enough that my GTD system works well within it. And that's really the goal, isn't it?