Matz likes the fact that it Just Works and you can't accidentally have weirdness based on how your code runs. There aren't any plans to remove it at this time.

I can imagine a whole bunch of gems suddenly breaking if this changes...

That's true, but I don't buy that argument: look at how fast everyone has updated their gems to be Ruby 2.0 compatible. The Ruby world likes running on the edge; if MRI forced us to consider thread safety, we'd learn about the issue much quicker as a community. Since we'd be forced to. ;)

Oh I don't doubt that the community would move quite quickly to support it, but it certainly wouldn't be without some bumps along the way!

The GIL makes a lot of things in the Ruby interpreter much easier to implement.