Nobody should be forgotten. Would you want to be forgotten in anyone's mind, regardless of who you are to them? So long as you know them, of course. I feel absolutely horrible when someone I spoke to so long ago starts talking to me and I have that cloud in my mind blocking who they are and then I have no choice but to ask who they are, despite how obvious it may be. Considering most of the people I've lost contact with over the years are people I met a while ago online and have gone through a lot with, there are things that just shouldn't happen, in my opinion, and forgetting someone is on that list for me.
Michelle pretty much summed everything about this topic up, as far as I'm concerned. You should keep everyone you meet close to you, regardless of how much they mean to you, especially if they're a good person. There are very few pure hearted people in the world and being able to keep one close to your hand is a very assuring fact that I think most people take for granted just because they aren't "friend" with the person. I, for one, make very few friends, and that is a decision I made on my own. But there are people I would like to get to know more. But just because I want to get to know them, doesn't mean that they'll become friends with me in the future.
So anyway, no, acquaintances should always be there with you in your mind. I don't think you should forget them, but sometimes that's out of your control.