The Microsoft of the VOIP arena?

My ISP (Nildram) recently started conducting VOIP trials, and I signed up. I was allocated an 0870 number, which can be dialled from the PSTN to contact me, and of course I can dial out, using the handy call credit Nildram gives for testing. The voice quality isn't bad, although everyone I've phoned has noticed the difference from normal service. So far so boring; nothing you can't do with an ordinary landline (although it's quite handy to talk and listen completely handsfree when I am working on the computer). The really interesting part of it is my address (really the same as the phone number without the PSTN prefix) on the Nildram SIP server. Using it, anyone using VOIP can call me for free, and I can call them, also for free.

Now when I said "everyone" of course I didn't mean everyone, because of course (there's always one, isn't there?) one player in the VOIP arena have set themselves up with a proprietary standard and a proprietary client application. I'm talking, of course, about Skype.

My friend has a Skype account, and the Skype client on his PC. But the only way I can call him from my Mac (short of routing the call through PSTN and completely destroying the logic of VOIP) is to download Skype for the Mac, get an account on Skype... Why should I do this? I already have a perfectly good VOIP client and a perfectly good SIP server. Why should I bother with Skype when they refuse to play nicely with the other children?