My BassettX has one. it was $40 with Homegain. It costs $15 a year to keep it updated and that provides insurance for any injuries sustained while they are lost. You don't have to renew - your info is supposed to be maintained regardless but they will notify all local shelters and vet offices for you.
I know from experience that that is hard to do while you are upset about them being gone.
Copper does not have one, but I should get him one.
It provides ID if they lose their tags so is worth it to me for the peace of mind.
They don't have to be under. I asked that Jack's be done while he was under for neutering, but I believe they forgot and did it right before I picked him up since his neck was et.