Every now and then you'll come across someone arguing that, instead of having several different desktop environments (Xfce, GNOME, KDE), their respective developers should team up to create The Perfect DE. Apart from the fact that this is just impossible, I'd like to argue that this fragmentation is actually beneficial. Benefits that go beyond "more choice".
Let's just assume that, all of a sudden, some magical mediator appears out of the void who manages to convince the developers of the several different open source desktop environments that are available nowadays to work together. Their goal: creating the perfect desktop environment and make open source operating systems kick ass.
In their quest for the perfect DE, the developers start to plan their new project. Apart from deciding what they will implement, they will also need to agree on how to implement it. Which toolkit are they going to use? Which programming language? Which version control system?
Obviously, this would result in endless discussions. Discussions that consume a lot of time. Time that could've been spent developing Great Software.
But even if they do reach a settlement and start coding, every developer will have had to give in on his/her (probably his :P) preferences. As a result, developing is less fun, which means the developers have less motivation.
What it should be like, however, and what it's like in reality, is that a developer works on a project he himself is interested in. This means this developer is more productive than he would've been in the situation described above.
Besides, competition is good. Different desktop environments will want to differentiate and focus on innovations. Projects can experiment with exciting new things while others can stick to traditional desktops for those who'd like to keep their habits.
Furthermore, with all this being open source, projects can easily make use of eachother's innovations and technologies. A perfect example of this is the relationship between Xfce and GNOME. Xfce's toolkit GTK is often being developed by GNOME developers, and thus innovations meant for the GNOME desktop also land in Xfce. And while Xfce, being the smallest DE of the three, might still be lacking in some areas, projects like Xubuntu can make up for the missing features by including GNOME applications.
In fact, you can also happily mix 'n match from all three desktop environments. Perhaps you like the customizability of Xfce's desktop manager
xfdesktop, need the network browsing capabilities of GNOME's Nautilus file manager, and prefer KDE's AmaroK as media player - it's all possible.
All in all, calling for unity in the DE world is quite futile. Besides this being impossible without an supernatural mediator, it would not even improve the quality of the software.