We do the same thing as JeremyE. With SQL 2012 you need the engine if you plan on using the SSIS Catalog as you now do all your managing of packages by connecting to the engine rather than the ssis service.
I like having all packages and related jobs on a dedicated server, that way it keeps the agents clean on my oltp and olap servers and I don't have to worry as much about a developer making a package that spikes cpu and slows down a production application.