Because the company I work for uses an Exchange infrastructure, I live in the MS world and thus it's my fault (or Microsoft's).
My Android phone does a better job at handling Exchange accounts than my mac. All my work environments are Mac environment, I even coded on open source OSX projects for a while so I am not the kind of person to just dismiss OSX because it is not a MS platform.
Like it or not, it is a reality that the Mail & Calendar are broken, the simple fact that Apple tries to convince everyone at each release that the new version is finally a good one is enough for me to see that they have trouble developing a good email/calendar clients.
And again, Exchange is one of the most used corporate infrastructure, that's the state of the market, and Apple should support it the best they can.