ff, according to your link, it looks like its free. i´m so used to those rob dogs ryanair, i cant even remember the last time i could put something in the hold for free.
Ryanair don't rob, they are pretty clear about their charges. If you want cheap flights you get cheap flights. Paying to put your bag in the hold just brings their fairs closer to rivals.
You can justify it depending on how much you paid for the flight. I've paid £30 for a return flight. Even if I paid the £35, which I've never done, it would still be cheap.