I disagree. Becoming friends with a nice woman is never a waste of time, even if they are attached already.
Attachments often don't last, and even where they do, she could potentially still be a good friend.
And Kero, if you have trouble talking to women, then the saying holds for this as well: practice makes perfect. Get used to talking to them, anywhere with anyone - even those you're not interested in (and thus you won't care if they ignore you). The more you do it, the more natural it will become, the more you'll find to talk about, and the more you'll learn about what holds their interest and what doesn't. It will also massively boost your self-confidence. Might even make you a few new friends! You want to get to the point where talking to an unknown woman in the street feels as natural as talking to your own sister (if you have one). Practice, practice, practice!
Cheers,
GP