I would normally expect 'typical users' to mean 'home desktop users', in which case MAPI support is irrelevant - who runs their own Exchange server?
I presume you are thinking rather about the sort of features that might encourage large-scale corporate deployment?
I kind of think this is leaving the realm of mail clients though. MAPI clients (without using nasty hacks, are there any other than Outlook?) aren't really the same thing as mail clients to my mind.
While Outlook is *technically* a mail client, it's about the worst you're ever likely to find (with the exception of Evolution once you start paying attention to things like performance and stability), but it's not really meant to be its function, per se, just one of its features. (It's interesting that Outlook does everything badly - but it does several things all together, which makes people happy)