Even though there are signs of a truce between these two giants, they have been incompatible for so long that people are not even trying anymore. Problems arise when you have to synchronize data between 'incompatible' devices such as Mac machines and Windows machines. Synchronization applications are essential unless you are willing to constantly copy and paste the data manually across different devices. Unfortunately, in the multiple gadgets environment that we live in today, it's almost impossible to always enter data to only one device. If you don't believe me, try to recall 50 phone numbers and emails of contacts that you've met in the previous month.
It's been proven over and over again that a normal human just can't rely on their brain cells to store hundreds (or thousands) of contact entries without losing most of them. This kind of data should be recorded the moment you encounter them, in your trusted devices, or they will be gone for sure. To some people, the most important data are contacts, calendars and events.
