I don't view it as "sinful", but I do view it as problematic. For one thing, I can't seem to tell whether I'm being flirted with or whether she's just being nice and maybe a bit gregarious. I think some women (and men) get labeled as flirtatious when in fact they are simply friendly. That's why I've soured on it over the years. Unless both people are definitely trying to send a signal that they want to date, I just don't see the value of it.
I realize that some people just want to flirt in order to put a 'feeler' out, to see whether someone might be interested. That's fine, but I think that, unless it's an unmistakable signal, it can be misconstrued. And worse yet is when friendliness is mistaken as flirting.