Not that this topic is really about loyalty -- I doubt anybody thinks less of Cowens for working for teams other than the C's - but I recently read a piece on Larry Siegfried, one of my childhood favorite Celtics that puts loyalty in perspective.
Siegfried played college ball for Ohio State who lost to Cincinnati in the NCAA championship game circa 1960. Subsequent to this heartbreaking loss he was drafted by the Cincinnati Royals (NBA). Refusing to play in Cincinnati, Larry chose to play in a lesser league (the ABL). It was actually this path that ultimately led him to the C's. Siegfried was contemplating retirement after the ABL folded the next year. John Havlicek, a teammate at Ohio State convinced Red to give Larry a workout. The C's signed him and he went on to play a solid role in five championships.
If I was not technologically impaired, I'd provide a link to the story above, but I don't know how (I'm old-ish). I believe it was Wikipedia.