i put bar-ends on my new '02 Specialized FSR XC which has a riser bar but not very much(maybe 1-2")... plus i have bar-ends on my old hard-tail now commuter which does have a riser bar.

i'm not sure how high of riser bars you're talking about, but i find the extra leverage of bar-ends necessary when climbing trails of 20% or more to keep the front end on the ground... when sitting you can move your hands forward, push down and crank. when standing i also like the bar-ends... i guess if you only do downhill or minor hillclimbs then they're not necessary. i've also heard the 'bar-end not necessary w/ riser' thing, but as far as i can tell, with the geometry of most XC bikes you still need them... downhill bikes is another story where i guess bar-ends are just another potential source of injury...

then for touring, commuting and longer rides i also like the multiple hand positions.