It does make sense if you look at the features page of Mountain Lion. The next version is not exactly overwhelming us with new features other than some implementations from iOS. It is rather meagre, to be honest. Giving away iMessage to other OS might reduce the overall value of Mountain Lion and the incentive to pay for it.
As it stands now, I would not be willing to pay for Mountain Lion simply because I absolutely see no added value for me yet. I dont use Reminders or Notes, hardly any of my friends use iMessage, I dont care about sharing stuff via the Internet or tweeting about it, and the Notification Centre is just not as a useful to me (frankly, I dont even miss it now). I can understand that many people may find these features good, but for me this just is not it.