To answer your first two questions: According to the Society of Professional Journalism one of the most important obligations for journalism in general is "To act independently", and without conflict of interest... Commentating for a team that you have a clear conflict of interest with shatters the whole idea of avoiding COI.
It's kind of a basic of sports journalism that you're not supposed to proclaim yourself a fan of your particular team and that you're meant to remain impartial. When a sports commentator proclaims (or makes it obvious) that they're a fan of one team or another it makes them lose a degree of credibility.
I EXPECT them to be biased as well, because they ARE. But that doesn't mean it's a good thing.
Honestly, I'd prefer every commentator in the league to be from out-of-market with zero connection to their team.