Yeah, even if you have a tri-band GSM phone, it makes loads of sense to buy a pre-paid SIM at your destination if you are going to make a lot of calls, especially if you are likely to come back. For example, a UK Vodaphone Pay-As-You-Go SIM costs UKP20 to start - so you do have to make a bunch of calls to make that initial charge worthwhile, but after that the charges are pretty reasonable (especially when compared to roaming) and there's no billing issues to keep track of. The only downside is that at the moment they don't have a pre-paid scheme that allows GPRS (though apparently WAP over GSM Data does work.) Don't know about other UK operators.
One advantage of having one GSM phone that works worldwide, even if you do this, is at least you don't have to worry about keeping two phonebooks in sync.