The thing with iChat is that it was always sort of a weak client for the fact that it primarily ran using AIM login credentials and never became as robust as Trillian or Adium - both of which allow for logging into a wide array of chat servers and have more features overall. Sure, Lion brings support for Yahoo messenger accounts, but it's still somewhat lacking.
Really, the only reason Apple used AIM for the server-side of their chat program was that, at the time OS X first came out, the company wasn't doing so well and chose to delegate a lot of services to third parties. In fact, at one time OS X even shipped with Internet Explorer as the default browser. Apple really didn't have its crap together back then. Today, iChat still feels a bit like old legacy software. A relic. It's in complete opposition to Apple's goal of controlling user experience from top to bottom, and everything in between.
Apple could leverage iMessage as the backend for its own chat service that it controls at all levels - from the client itself to the server-side implementation. Add Facetime to it, and you've got something that's pretty substantial as a service suite that can compete against the likes of Windows Live Messenger.
Apple needs to find a way to gracefully integrate iChat, iMessage, and Facetime into one application. Unfortunately, we won't be seeing this in Lion, but perhaps we will later on down the road.