I find that most of these are overstated though. Uptime, in particular. What's to say your server will have better uptime than Google?
If there is a downtime on your self-hosted platform you can decide how much effort/time/money to spend to fix it, depending on how urgent it is to you.
If your email goes down, and you need to access an old email RIGHT NOW, you have numerous options; get in a taxi NOW, go to data centre, remove hard drive, pull of that one file that will save the business. Or hire the best person you can find for a very short term contract. Or if the email isn't important you can wait till the morning or whatever. With Google hosted email, no matter how much money you have available, you cannot get it fixed quicker.
Self-hosting means if things go tits up, you can have some say over how quickly it gets fixed.