For starters I have too many friends and business colleagues who are NOT ON SKYPE. Enter Palringo, a client, and an aggregator of IM networks that does more and easier messaging for me. Plus I can use it on just about every mobile I have except my Android, if I want to.
Next I won't abandon SightSpeed as it isn't an IM client, but a video conferencing app that is perfect for Group Video calls, going beyond the one to one capabilities of Skype.
But....and this is the big BUT, I can see getting rid of more than just IM clients as Skype and SIP based services keep doing more for me than traditional voice platforms do.
For example, I can't get numbers from my local phone company that are based in foreign nations. I can do that with Skype and can point SIP based DIDs to my SIP service from OnSip or inPhonex or CallCentric.
What's more, I get tons more flexibility with capabilities.
So, at the end of the day, it's not about giving up only IM as Alec points out. With Skype and SIP it's about saying later to your old landline.