The problem is that MLO isn't a calendar tool. For example, if you receive an invitation to an event or meeting, in the open and widely used vcal format, you won't be able to add the item directly into your schedule in MLO. Likewise, you won't be able to send other people appointments.
I understand that you want to simplify and minimise your sources of information, but I think that you should still use the right tool for each job and in this case I think you'd be going too far in that quest for simplicity.