I must say, I agree absolutely. If she's the type to cheat or if she's loyal, you may see that in this situation (and weed out a bad egg) but if she's with someone she senses isn't the right one for her and it happens that you are, she will sense it and act on it. What can it hurt? Give her the option.
My hubby and I were both seeing someone when we met. We both were not committed, openly looking, but only seeing one person. After we went out, we both informed our the other party that we found someone we want to date seriously and that was that. I knew the difference between Mr. Right Now (I had fun with that guy but already knew it wouldn't last the summer) and Mr. Right - my husband (who is wonderful by the way! Since meeting him, there's been no question and I've been good to him and he to meet. I agree, go for it. Besides, a girl likes a man with balls who knows what he wants. :) Good luck