I think we all like to see new birds; now if that bird is a local or national rarity, travelling to see it is twitching. Therefore, I have twitched - and I hope to live long enough to twitch again. But, I also enjoy seeing commoner (at the moment) birds on my local fields. The twitches I have been on have enabled me to meet some really nice people, and some fellow-weirdoes. The manics chase the bird frantically in ever-decreasing circles (though no-one has yet reported an Oozlum bird), and the rest of us stand and chat while waiting for it to re-appear. As has been said, one criticism is that twitching leads to mis-identification, relying on other peoples' id skills, mass-hallucination and over-confidence. My own rule is that, unless I see the bird well enough to be confident of identifying another if I was alone, I don't tick it, not even in pencil. I've not yet seen Wilson's Petrel well enough for it to be on my list - but my list is for my own enjoyment, I don't feel I am in competition with anyone else, and I work to standard BOU lists. I have had rarities rejected by the BBRC, which came off my list, even though I was certain of the ID at the time. Mind you, twitching by public transport adds a further element of uncertainty to the pastime - it's not only will the bird still be there, but will the bus come ? On one occasion, in Dorset, the bus driver went a mile off route to drop me closer to Winspit, which I thought was very nice of him, and the other passengers all wished me luck as I staggered down the road. On another occasion, I diverted a coach-load of club birders to see a Little Bunting that was 'almost' on our way home. Birding's fun - so's twitching.
Tony