I agree that using Skype to call phone to phone is kinda pointless - saves minutes, I guess. Calling Skype to computer (or vice versa) has its uses, but I think one big point is largely being missed here: if you have a Skype account and SkypeOut credit, you can use Skype on your iPhone to call regular phones anywhere in the world for pennies a minute (depending on the country). Not that most people do a lot of international calling, but when you need it, it works great.
IF you're calling international - useful.
If you're roaming in a foreign country - it becomes extremely useful. Anyone on holiday - anyone not getting a local SIM card etc. That's a big chunk of people.