SMSs are more similar to emails: they are asyncronous. IM is syncronous. You can emulate IM using SMSs but it isn't the same thing. It is more similar to exchanging a series of emails. BTW SMS runs with very bad (GSM) network coverage, too.

Read this article from RCRWireless - ' TEXT MESSAGING HAS BECOME A CASH COW for mobile operators desperate to generate revenues in mobile data. But instant messaging may be poised to change that.' I personally don't like instant messaging - I use it sometimes, and I do find it very useful...