I used to try to keep collars and ID on my indoor cats b/c, after all, stuff happens and I didn't want to take even a small risk that they might get out and be out there w/out ID. But I couldn't get them to keep the collars on! It is, of course, unsafe to put anything but a breakaway collar on a cat, and it's just too easy for them to get out of them. Since I couldn't keep them on the cats it seemed pointless.
Do be sure that your cats are microchipped!!! That way, if one DOES get out and someone finds him/her, you stand a good chance of getting the kitty back.