I'm not sure if their stance has changed and I can't seem to find the answer that was given, but it had to do with aesthetics/layout. Basically their web layout width was too restrictive for showing side by side diffs. They only really have two options:

1) Do what Bitbucket does and have the side by side diff be a pop up window. I personally find this to be jarring and unnatural.

2) Combine both elements. It's what I do for my product. To better understand this take a look at

http://ny.testdrive.gitsense.com/index?#pid=22&cid=27&trail=

and switch to the side by side view. If you hover over the line number, you'll be able to see the diff line in unified mode below. With this setup they can provide side by side without having the user to constantly scroll left to right.

Aren't they complimentary? You can have a side-by-side diff without the extra context this change introduces.

That said, I also want side-by-side diffs. They're necessary for more complex diffs.

Agree with the adjacent post that this is a complementary feature request and I also really want it. There is a browser plugin that can sort of get you there, but it's pretty brittle.