Personally, I think there are a few reasons for it:
1) Showing off. The one reason I liked mc360 so much was because it was the 360 dashboard. So for me it was nice to show people, "yea... thats the 360 dashboard on my Xbox1.", "yea... thats the AppleTV interface on my Xbox1.". No matter WHAT those interfaces looked like (Beautiful or Ugly) they were going to be skinned to xbmc. Thats why
some of us like them so much, to show off.
2) Easier to build. Instead of constantly thinking about the most perfect layout, how everything will look, etc... 'Skin Clones' (MC360, xTV, etc) already have those layouts made. The only work needed is creating the images and xml... Im NOT saying its an easy job, but a lot easier then coming up with an original idea for a layout thats never been done before.
3) XBMC Features. What I enjoy most about Skin Clones is the ability to have the same exact look, but take advantage of all of xbmc offers (Everything the original does and a lot more). So for the people who think, "oh wow AppleTV is nice, id love that", you have XBMC with xTV. Which does about everything AppleTV can and more.
For me, I have
always felt the most important part of a skin/gui is the
layout. Thats the key. After you have a nice layout, then you'll have themes, and other skins based on the layout... With Vision, I spent so much time thinking of a creative unique layout that hasn't been done, I had some crazy mockups... But in the end? I chose Project Mayhem 3's main layout. You just can't beat it its perfect. You'll notice I added my own idea's like Hidden Sidebar, Topbar's Folder.jpg, etc... But its mostly PM3's layout. Except for Visions home. Which is the Vision I had for it since I started it.
As far as XBMC having its own unique look... Thats PM3. Like I said, its pretty hard to top such an amazing skin... but I hope it gets done soon, XBMC is due for a entire new unique look that takes advantage of XBMC more then PM3 does. Who will take us there? Well theres great talent that just joined Team XBMC, I'm sure those creative minds will think of something amazing
I'm sure these reasons don't apply to everyone, this is just my personal opinion! So please dont' attack me with cat pictures!