In reply to goatee:
I would say, as long as you're quite frank about your limitations so that the other person has a fair idea of what you can manage, let them decide for themselves what they feel. You never know, you might give someone a boost by being the only person they know who climbs less well than they...or they might want to build up a variety of experience...or they might just want a chilled day out on the crag and not care how much climbing they get done...
Don't give yourself such a kicking for every failure - enjoy the moves you can make, learn from the ones you didn't make this time, imagine how you might make the moves next time.
And no decision not to climb needs to be final, why should it be?
(I might be slightly biased, I've been in a state where I could barely walk - friends who would come out with me regardless made a huge difference to my mood and faith in humanity.)