I've used them for many years and found they gave me more hill climbing power when I used to stand on the pedals. I don't do that quite so much these days but I would always fit them, as you say they give a rest and change of position to wrists.
As Flecc says they make up-ending the bike a lot easier and safer.
I would advise getting the ones that aren't integrated into the grips or can be adjusted independently of the grips, the angle you may like for the grips may not suit the angle you like for the bar ends.
I can very strongly recommend Flexiseq for arthritis, it really is effective for myself and others I know, the relief after using for a few days is unbelievable!
Not cheap but worth it and lasts a while.
On offer at the moment from LLoyds on line.
http://www.lloydspharmacy.com/en/flexiseq-gel-50g-twin-pack-423601
I'm quite wary of claims made for miracle treatments but this really has worked for my knees and wrist.
Dave.