Managing the address book can of course be done with another application, but if the email client cannot use it, it's no use even creating one.
Then, once you have all your contacts in an electronic address book to use from your email client, you don't want to enter all your contacts again in your smartphone, so you need to store your address book on a server using a widely supported standard. This is even more the case if you need to share contacts with other people (e.g. your colleagues, your wife).
The same applies to appointments: you may want to manage your calendar with another application, but scheduling appointments is done through email messages, so you email client needs to interact with your calendar server.
Because of course, I want to use the same laptop both for home and work usage. I'm lucky enough to be allowed to choose the OS I run as long as my work is done, it would be a pity not to jump at that chance... 😉
