The problem with relying solely on "real world makefiles" is that there is not a lot of variation amongst those that are publicly accessible. The vast majority of open source projects that use make are autoconf based, which means the makefiles are all more-or-less the same, except that the list of source files and the project name change. Once you can successfully build one of these, you can probably build all of them.

There are a handful projects that don't follow this pattern, such as Mozilla, but once you've built those they cease to be useful for flushing out new bugs.

So, what do you do once you've built all the open source projects you can find, and you are still finding incompatibilities in your parser when you go to customers? Markov chain-based makefiles are a reasonable way to extend the breadth of our testing. Obviously it's not the only thing we do, but it is another valuable tool to add to the toolbox.