This topic has come up any number of times, but I'll let you do the search(es).
There's been a lot, I mean a lot, of bluster about how cool you are with more things on the stick. To paraphrase what I learned at Boeing, functions have to earn their way onto the stick.
Qualifications for a switch to be needed on the stick:
* Needed in a hurry, like PTT, or autopilot disconnect, in case you see a thermal full of buzzards ahead. This is THE most important factor.
* Used often, like PTT or elevator trim. Secondary.
Disqualifications
* Anything where inadvertent actuation can cause a problem.
Don't forget that there is very little standardization on buttons on control sticks, so your solution may be a setup for the next person who flies the plane to get in trouble, or for you to build bad habits for the next plane you fly.
I didn't build my RV-9A, and the stick only has PTT. Putting other buttons on the stick would be a major, major pain. I've got TO/GA on the panel (works fine), autopilot disconnect is on the panel (see above comment about buzzards), and elevator trim is a rocker switch above the throttle. That would be okay were it not for the fuel pump switch right next to it which is hard to distinguish by feel alone, so that could be a good solution but isn't because of the neighborhood.
And it's also interesting that when it comes to human factors evaluations, people can fool themselves quite easily. When the Macintosh was being developed at Apple and they wanted to evaluate the mouse, they ran timed experiments of mouse vs arrow keys. All the subjects swore that the arrow keys were faster, but the stopwatch always voted for the mouse. Similarly, when well-intentioned folks talk about how cool it is to have all those buttons on the stick, there's a very good chance that they're fooling themselves...