In my opinion other than maybe matching some other parts on a bike they are absolute rubbish whatever bar/bike they are on.
I had a set on my old Peugeot for a few years (because some of the pros had them) and found that:
a) they hooked every bush in sight on some of my local trails to the point where they caused one too many stacks.
b) flat bars in most cases are too narrow for good control in the tight technical descents and bar ends effectively force you into a narror grip which makes the situation worse.
c) unless you are on a bike with a stupildly short top tube they don't aid your climbing. A mate at the AIS can present a better evidence on this point.
When I took them off I was amazed how much more enjoyable my riding became with the tiny bit of extra bar length and the ability to brush bushes without being flung off.
Bar ends belong where there are for most of us currently either hanging on a nail in the shed, or on the bike of a roadie pretending to be a mtber.
I think Jez may have hinted previously that there are many bikes on here with them - although I haven't checked photos for evidence
Edit: Only 2 out the 16 bikes nomitated for bike of the year have bar ends. I rest my case