Yes, true, but a heck of a lot cheaper than roaming charges particularly data.
3 definitely score if you're already a subscriber, or doing a tour through more than one country. It's also a pretty reasonable deal if you only buy as a one-off for mobile internet for a shorter trip.
For calls, if you aren't already a 3 subscriber it may just be cheaper to roam with your existing provider - because you may end up paying for credit that you don't use. If you go on a long trip or to the same country a lot then it may be better to get a local SIM, but only if the credit you put on the card doesn't expire.
For data on longer trips it may be better to get a local SIM - in Italy for example 3 has pretty good coverage but Telecom Italia was a bit better.