And I wonder how many companies have benefited from the experience said programmers gained while working on pet projects...

I look at it like taking a walk, or a smoker taking a smoke break. That small reprieve, the 15 minutes I take working on a side project, or tinkering with something for fun, rejuvenates me and gets me jazzed about the task at hand when I get back to it. That's a productivity booster, and ultimately a benefit.

A rationalization that's likely to get you fired if you were to own up to it publicly. Although it's true, the company would benefit from you feeling excited about the task at hand.

Probably a lot, but despite the title, that's not what this article is about.

People are being pretty friendly to this idea but honestly I've known more than one disgruntled developer, on their way out, who basically worked 90% of their time on their pet project, just doing the minimum work to not get fired. I don't think that is what people are advocating for but to answer the original question, I think this happens a lot where it isn't "practice" that the employer benefits from.

I would just view pet projects as practice time. Its like getting mad at a musician for playing when the mics are turned off.

Well, doing it on work time is more like getting mad at the musician because they're supposed to be in the next room recording a track...