That perhaps proves the point, albeit a slightly 'clunkier' interface, it's never caused a problem for me (so much so that I didn't even realise that it was different ^^. I can still book meetings, read email etc etc so it does the job.
I'd be interested to hear which functionality didn't work.
firefox on 10.5 @ home and OWA at work has always worked fine for me. This includes calendar, mail and tasks.
Perhaps you're using some functionality I don't use from home?!
Initially to keep the technologies the same for simple management and making it easier for users to switch OS. PLESK has a Linux and Windows version, until CPanel complete Enkompass, the IIS version of their control panel then it makes sense to use a similar commercial product which works well on both OS's.