The simplest way to put it is that the Outlook MAPI subsystem cannot handle the requests like the MAPI subsystem installed by Exchange.
That is why RIM does not support it, you may not get MAPI issues for a long time but you will get them. Once you do, they can only be fixed by removing Outlook and installing ESM.
Keep in mind of the MAPI versions, Outlook = 1.0, ESM = 6.5.
It's like putting a 3000lb car on a jack designed for 1500lb. Eventually the car will fall on your a$$