A guy friend of mine recently started calling every female friend of his "sweetheart" and "darling". Gah! Here's my opinion (again!) on people shouldn't do this:
It's overly familiar. This is the sort of language used to talk with a partner. People don't just get to be familiar with another person because of some combination of their genders. People have to earn that privilege. To behave as though someone is close to when they're not is to presume that you already know what they are comfortable with, which is a dangerous assumption to make.
It's patronizing. It's something you might use to talk with a small child. Need I say more?
It suggests entitlement. Entitlement of my affection, or entitlement to receive their affection. Which is hella creepy.