I've always thought that they had a lot of benefit. As Double mentions, you can get much more training in without beating yourself up as badly. More training, less wear and tear. Seems like a winning combination to me.
Of course, there is some limit to the benefit of them. Are you better off running 2 in the morning and 3 in the evening, rather than 5 all at once? Maybe if you're a miler and quite a bit of that running is done fast. If you're a marathoner, you're probably better off getting all 5 in at once.