Performance is one reasons, a single point of failure is the other. If your server crashes, you not only lost your exchange data, you potentially also lost all of your backups as well (if you use only B2D).
A lot depends on your environment as far as performance goes, but there is no way around the single point of failure issue. Even with tape backups, you would still need to rebuild your server, reinstall BE and recatalog all the tapes, just to bring back your exchange server.
It would be nice to keep them separate, but it's perfectly feasible to have them on the same machine. If you are just doing nightly backups, when Exchange is being little used, performance shouldn't be an issue.
I have 11d installed on my server as well as Continued protection server (CPS). I wanted to configure a CPS backup job and it would not let me do so. It stated "There must be at least 2 servers ...." So I am assuming that CPS cannot be loaded on the same server as Exchange due to the nature of it's feature. If the server goes down the continued protection backups would be lost as well?
Yes, there must be two servers for a CPS job. This is because the Destination Location (for the CPS backup) cannot be on the same machine as the main CPS install. The reason, you can't do a CPS job on the local machine since it would be brought to it's knees if you proteccted it. IE: Replication of the replicated data would be a continuous circle...
If you are doing CPS Exchange jobs, you would want to install BE and CPS on the same remote server, then push both agents to the Exchange server. (RAWS and CPA)