Please understand that 15K is a fraction of a fraction of the number of MSN Messenger users on Windows. Also consider that the Mac division is separated from the Windows division. Then there is also the fact that over the last decade or so, Microsoft has concentrated on apps and tools for their own operating systems.
For about two decades, MS made available more titles for Mac than any other software company (probably still does). And they almost abandoned Mac on Office about 9 years ago, but didn't...even tho' the monetary return was far less considering the resources devoted...a lot of money was spent for a fraction of the PC community.
The latest version of MacMessenger at least has some necessary stuff...and it definitely looks prettier than its Windows counterpart. Features are nice, but functionality, at a shoestring budget, is more important.
Remember: features in MacMessenger 4.0 is currently way less than what v5.0 was for Windows. And v6.0 for Windows was released almost two years ago. You're asking for four years of development (eight years in Mac time) for a client you expect now (or expect in less than a year)
Ain't gonna happen.