People say that a lot, because they've heard it somewhere, but do you actually have the facts to back it up? I'd have to check my books to be sure, but I think you're talking about SMS delivery over the SACCH. That's an
associated channel, meaning it goes along with a traffic channel, so that would apply if you were in a call. If you were not in a call, it would have to go over a standalone control channel SDCCH -- in other words, that channel would be being specifically used for the SMS transfer.
And, of course, there's the cost of running the SMSC.