What's the point of playing in a league that bad? I understand the "keep in shape" part, but wouldn't you try your hardest to find a spot in a league that could possibly help your development?
It's not that easy for a guy of Bonino's caliber. While he'd improve teams in many better leagues, most European elite league teams don't use their usually limited spots for imports for players like Bonino, and save their money and the slot for higher-profile guys, or wait until the season's cancelled entirely. For guys like him or Beleskey, there's probably just no spot available in a top league. And after that, it becomes a bit of a crap shoot - I'm not sure the players or agents really know how the secondary and lower tier leagues in Europe match up against each other. I agree that it obviously doesn't do anything (meaningful) for his development, but who knows whether there was a gig available for him in a spot that would even somewhat allow for that, like in Allsvenskan, Germany's 2nd league, or the likes. The market's pretty limited for him.
I won't disagree about the level of competition Bonino is facing. But in looking for a positive to it, he sure is gaining a lot of self confidence, even if it is against lesser opponents. Sometimes a little more self confidence can go a long way. Guys sometimes get sent down just for that reason.
I also think that playing real games, even at a lower level, is better than just practicing with better players. YMMV.