Got to say Inky, I'd never let mine outside without their collars on. Cats wander rom time to time, and not everyone is clued up enough or (sorry) HONEST enough to get the cat scanned for a microchip.
Case in point - last year a friend of a friend found a cat wandering around a building site. She watched it for a couple of days but it was obviously lost, so she took it in to feed it but didn't know what to do so contacted me.
FIRST POINT - she didn;t know what to do. If it'd had a collar with a tag, one phone call to the owner would have sorted the problem. At this point, I took the cat to the vet to get him checked over and scanned - no microchip.
SECOND POINT - at this point, some less thoughtful or considerate people would have kept Alan (as we called him) - he was a beautiful and lovely cat and, hey, it was tempting!. Again, with a collar we'd have known he belonged to someone and wasn't feral. However, we took him home to care for him and put some posters up around the building site where he was found and in local vets and shops. It took FOUR DAYS for the owner to contact us, and I believe that was somewhat fortunate. She was worried out of her mind and he'd been missing a week. Needless to say she was very grateful and we told her she MUST get him chipped straight away.
THIRD POINT - peace of mind. Alan's owner was worried sick for a week. If he'd have had a collar & tag it would have been sorted within a day or two. Having experienced this, for my own peace of mind, mine will always wear a collar when outdoors.[OUCH!! - Sorry, Lilly's sat by me and just nipped my fingers! haha! She says she agrees with me.
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I take the points made about cats occassionally getting caught in their collars. But this is a very rare occurence. Yes, it could happen to mine or to yours if they wear a collar. But it's unlikely. Mine didn't like their collars at first either, but we persevered. We put them on indoors and kept an eye on them. They struggled at first, but it didn't last long before they got used to them. Equally, they do slip their collars occassionally - we've lost a couple over the course of the last year. But you can get a very good safety collar for about £6 - it's not a big deal.
Check these out:
http://www.thepetextraordinarium.co.uk/index.asp?c_id=2&s_id=11. Ignore the flashy ones on the first page and look at all of the ones with the safety clip. They're the best we've found, and I can tell you, we've searched and searched!! I trust these on my own cats, we sell LOADS of them and so people in general trust them too. You can get a tag to match here:
http://www.thepetextraordinarium.co.uk/index.asp?c_id=2&s_id=36 and we even engrave these free of charge.
At the end of the day, it's your decision whether to keep a collar on your cat. But given my experience, I personally think there's no decision to be made as the benefits FAR outweigh the risks.