I'm more inclined to point towards the health of the yeast. I've had a few beers do this, both ale and lager, where it doesn't hold a head and goes flat looking. Fermentation temps were good, and were kegged and chilled immediately following fermentation (2 weeks or so). But I guess I'm just spit balling...
I don't think it's necessarily letting the beer sit at ambient temps for an extended time. Either way, it doesn't happen all the time, so it's really hard to say what the cause is. I guess, like Frankenbrew said, if it tastes good, what's it matter? There's lots of commercial beers that do this as well, especially a lot of the American lagers (all of them, really...). What's the cause of the head falling in those beers? Adjuncts? Doesn't seem likely, but I don't know...