At TitanFile we run our own server for all application-related stuff, and use a third party for email hosting. Reasons:

1) We need to comply with Canadian privacy laws, so need Canadian VPS - and those are way more expensive than in US.

2) To be PCI compliant, you need access to server logs. You can get that with Rackspace but pay a premium.

3) I've managed an in-house data centre before on my own, so I know what I'm doing for the most part.

If you factor all of the above, it's actually cheaper to manage your own servers, even if you outsource the sysadmin stuff to a competent contractor at about $100/h. The catch is there is an overhead initially to provide redundancy that diminishes only when you can utilize that extra capacity.