So yes, I paid $50 per Mac so I could take advantage of full-screen mode with multiple displays. Earlier this year I felt compelled to upgrade from 8 to 9 because full-screen support in v8 was terrible.
To be fair, they usually do provide a final release that allows compatibility, but you're right, a new major release is required to take advantage of new host OS features. Would be nice if one could keep using a piece of software for more than a year. Every Year when Apple brings out a free OS upgrade, parallels stops working and suggests buying the latest version for $50.
