I'd like to comment that no matter how hopeless or pointless this seems because of the lack of practicality, there's still no harm in discussing an ideal, because all things I believe should come through an ideal first and then determined for practicality second. It makes living a lot more interesting.
I believe unrequited love is silly, but only because it usually involves a simple crush or infatuation with little to no actual experience or interaction. It's tragic otherwise because the term implies that you're doomed after everything you've tried has failed.
Otherwise, if there is even the slightest chance, I think that someone should go for it. Love itself is selfish, and to deny a selfish desire a chance is going to hurt no matter what, because it's basically a chemical dependency that needs to be killed off by other experiences. The entire idea of thinking that someone will be happier and better off without you, I think, is just plain stupid, because why would you cause yourself so much pain? I've heard my friend's ex-girlfriend use that as an excuse to break up - "If you really love me that much you'll see that I'm not happy with you." To be done that by someone else is just mean and not a great excuse, and to be done that by yourself is just plain denial.
Of course, there are circumstances... but by then, you should have had enough maturity and had enough negative experiences involving the other person to realize that it's making you extremely unhappy by continuing to think that way. Thought should always come before action... and although I may sound definite, I really am not. Things for each situation are entirely different and no one solution fits all, one just has to review experiences and come to a better understanding that affects you positively... or end up in depression.