Sure, some valid points there, but to play devils advocate.. Say you are on Office365, If you have a thirty person office and a single person sends a 10MB attachment to an internal distribution group which feeds fifteen mailboxes then all of a sudden you have 160MB of bandwidth being used for a single email.
As for a recent horror story, I had a client come aboard recently who already had an in-house RDP server supporting 10 users. These 10 users all had access to the *same* four mailboxes all hosted on Office365. They receive a lot of picture attachments as part of the job, and like to file these received emails in a strict folder hierchy.
Literally every large email that came in had to be downloaded ten times. Then someone would file the email, and a big resync would take place. First thing we did was convert that RDP server to a VM and upload to a remote server.
Generally I like hosting email servers on fat, remote pipes, but when the client doesn't have the bandwidth its not always the best solution.