I think it has to do with the fact that they are not from the sea and therefore likely have a lesser amount of Omega-3 fatty acids in relation to Omega-6 fatty acids, but that is just a guess. I have fed them to my butterflyfish before and they did great with them. I did soak them in Selcon first though. The way I see it, if that is all your fish will eat, something is better than nothing. I would also be interested to know if there is any other real reason why these should be avoided with marine fish.
I've always used them as part of my tank's diet, although I rotate between a dozen different foods. My fish love them, particularly butterflies and angels. I agree with soaking them in supplements first.
My supervisor at LBAOP always had me feed the butterflies in QT with bloodworms. It's the best way I've seen to get the picky ones to start feeding. I don't think it's an appropriate sole feed, and as long as you don't overfeed there should be no problem.
I change up with pellets, mysis shrimp and blood worms for a diet. sometimes they get all three and sometimes one each and so on. I have herd people say that it is not good for he fish. The pet store was qiestioning as to why and notto feed them it. I use it for just the protien to help supplement the diet.