The only thing i can think of that would be a problem would maybe if you were adding bleach or something, then it would be getting dumped straight on the clothes. Or maybe front loading detergent is stronger and wont get diluted before reaching clothes as it would if you put it in the top? I doubt it though.
I would venture to say it won't do any harm, but I'm not sure it will get your clothes as clean all the time. Front loading machines use about the half the water and a different method of cleaning (removes dirt by working the detergent through clothes using a drop and tumble process versus rubbing the center agitator). Because of that, FL's require what's called a high efficiency detergent formula -- the average amount of which might be too diluted in a top loader. I admit that's just a guess, but I don;t see any harm in it. :)
