Well, to each his own, but here's why I said what I did:
My own understanding of ergonomics is that the closer your hands are to neutral or "handshake" position, the happier they are over the long term. The really expensive ($200-$300) ergonomic keyboards don't rest flat on your desk, they are raised ~90 degrees so that your hand is closer to the neutral position than a standard keyboard will allow for.
If the "neutral position" theory is valid, (I'm not claiming to be any great expert here) then look at what you're doing with a mouse: you're resting your hand flat on your desk (in a "pronated" postion). A thumb-operated trackball, however, allows you to keep your hands in a "handshake" position. I use my own trackball in that position and it seems to work well: when I put it flat on the desk again, it's still comfortable, but not quite
as comfortable when used in a neutral position.
Getting back to wrist rests: personally, I'm uneasy about the prospect of resting my wrists on a pad for any length of time while using a mouse. Yeah, okay, perhaps my fear is laughable, but if Carpal tunnel syndrome is caused by the median nerve (which runs through the wrist) being pressed or squeezed, using a wrist rest doesn't strike me as being the best idea.
I guess I'm not clear why, if the OP has been using a mouse without a wrist rest, he feels he needs one now. I mean, it's none of my business, and as I said in my OP, maybe it is
a good idea after all, but the ergonomics just seem all wrong.
A wrist rest reminds me too much of people buying those cheap-o drug store wrist braces to get them through the day. If you need some sort of "brace" for your wrist, maybe you're doing something you shouldn't be doing with it. I don't know.--
"Ah father, that sermon today.......frankly it bored the arse off me."