Mine aren't done for the following reasons:
1) I don't see my buns ever escaping.
2) If they did escape or were let loose I think you'd have close to zero chance of catching them
3) If my buns went missing I'd put posters up, get in touch with the local vets, put it in the paper, phone the rspca etc. I think that gives you virtually as much chance of getting them back as chipping.
I suppose if they were stolen that's when it would be really useful, as you can identify them as your own. But that's only if you find the people who took them.