Can you give an example of where it does make sense?

I'll venture a guess that you probably think that it made sense for Digium to open source Asterisk. At first, it did seem to make sense since they made the telephony cards that it worked with the best.

However look at them now. Sangoma, Pika, NxtVox and many other companies are making cheaper telephony cards that work just as well as Digium's do. Now what does Digium do? They start selling a closed source version of Asterisk that has more features in it than the open source one.

Open source is really nice for users but not so much for long term business profits.