Of course not, they shouldn't have to. If the N900 skype module wasn't so limited, it would be able to talk to older Mac/Linux clients exactly as the windows client does.

Imagine if MS Word 2007 had only read & written .docx files and not been backwards-compatible with the .doc format. Cries of "Microsoft can't make anyone upgrade" or "problem isn't with Word 2007" would have sounded a little lame.

There will probably be financial or technical reasons why the N900 client is so limited in its compatibility (as I've speculated) but we haven't been told them.