Fragmentation is not a bad thing. Think of it as natural selection in the open source software world. This is the mutation that may result in a new or different product.
My first reaction to the parent post was identical: diversity must be good. However, thinking about your evolution analogy I realized: if you really want to wait for (tens of) thousands of years for a piece of software to evolve from "ape" to "human", than simply waiting for the the natural selection to happen is the right thing to and eventually it will bring us brilliant software. However, this approach also fragments the community and diverts efforts from forward-thinking innovation to saving some dying technologies. There is nothing wrong with supporting projects that choose to stick to the good old ways -- by the end only the fittest will survive anyway (and that's called conservativism). But let's be honest, by building on Gnome 2, MATE is investing effort in taking steps backwards. Now I'm not saying that what they do is worthless (in fact I hate the guts of Gnome 3 and Unity), but I would still argue that just because a considerable part of the community doesn't like the new direction, development effort should not be invested into paving a road that is a dead end.
That's why on a second thought, I think it might be better for the community that the effort goes into e.g. Cinnamon, or why not a Unity fork.
One more thing: the GNU Linux ecosystem's great diversity is often mentioned as a great advantage. What leads to this diversity is the very thing we are talking about, fragmentation. A high level of fragmentation results in a bazillion choices and greater choice is for the greater good, most would think. However, psychology research suggests that choice overload can in fact be highly detrimental:
"The more options there are, the easier it is to regret anything at all that is disappointing about the option that you chose." (Barry Schwartz)
If you fancy taking a dive into this topic check out Schwartz's TED talk (http://www.ted.com/speakers/barry_schwartz.html) or his book "The Paradox of Choice: Why More Is Less".