The important thing about email and SIP isn't that you can change providers on the drop of a hat -- indeed, in both cases, you're generally at the mercy of your provider's technical acumen when it comes to scaling, uptime, etc.
The important thing about email and SIP is that there are lots and lots of providers, and they interoperate. So if your current provider sucks consistently, you can get fed up and switch to one that works better; because there are lots and lots of providers competing, there almost certainly *is* one that works better, and because they interoperate, you can switch even if your friends don't. (You can even run your own registrar and switch to that.) With Skype, everyone's pretty much stuck, and at the mercy of the one company. They might do pretty well as companies go (for now), but it's unlikely they'll do as well as any company that managed to climb to the top over heavy competition -- why should they bother, they're making money now.
(See also Jabber vs. proprietary IM protocols, and any article on vendor lock-in...)
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