People like options, especially the Linux/FOSS people. You could come to a consensus about something but it'll only go so far until some developer decides to fork it and make their own project :)
Yet another DE means extra set of conformance tests, bugs to fix, look-and-feels, supporting pages. These all mean money but your customers may not even care about this feature.
So let's say GNU/Linux can use whatever DE and it will always be. But RHEL is just a business product derived from it, with a selected "blessed" packages. There're no conflicts.
If Windows were open source I bet you would have people forking Win7 or WinXP and many non tech people use the familiar thing then be forced to use the new shiny thing some designer thinks is best.
Even developers likes to focus on their work and not why their computer doesn't start again after being suspended. How many developers uses Macs? I have meet sysadmins that uses Time Machine instead of a more customizable back up system. Even Miguel De Icaza, the co-author of Gnome, switched to Mac because "it just works".
Yet another DE means extra set of conformance tests, bugs to fix, look-and-feels, supporting pages. These all mean money but your customers may not even care about this feature.
So let's say GNU/Linux can use whatever DE and it will always be. But RHEL is just a business product derived from it, with a selected "blessed" packages. There're no conflicts.