Of course there won't be iMessage for Mac. We have iChat. No speed reason to have it.
iChat has video chatting features, but we still saw Facetime come to the Mac.
The thing is, although feature-wise iMessage also duplicates something iChat currently does, you have to take into account the fact that it interfaces with iDevices in a unique way bridges the gap between iOS and OS X. Also, Apple put in the time for R&D to create the back-end server and messaging protocols that run iMessage, so you have to figure they're going to want to deploy it as much as possible to get a return on their hard work.
Plus, the fact that iMessage is meant to allow non-phone devices like the iPad and iPod Touch communicate with iPhones shows that it's meant to be more of a universal communicator rather than strictly a replacement for MMS/text messaging services. Therefore, why not extend it to the Mac as well?
So I'd argue the complete opposite. You might very well see iMessage as a standalone app soon, or integrated with Facetime (although the name of the app itself implies it's only for video chat, so it's not a good idea), or you might see a convergence of iChat, iMessage, and Facetime (probably the best solution.)
We've seen Apple trying to cross-port as much stuff as possible lately. I mean, iWork for the iPhone almost seems ridiculous on such a small screen. However, they did it anyway. Apple is committed to narrowing the rift between iOS and OS X. Part of the process of shifting toward a "post-PC era" means allowing PCs to communicate with the devices that replace them during the interim and creating parity between feature sets.
I'd bet decent money that you can expect to see something pop up on the Mac App Store in the next few weeks.
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