Sponge in the diet is probably important, but I would be more concerned about ensuring that your angels get some green stuff. I have a Regal Angel that eats both ON Angel Formula (which has sponge) and Formula 2 (which has lots of green stuff). He's a picky eater and doesn't eat much else (but see below). While I'm sure each species, and individual for that matter, has its preferences/requirements, my Regal goes nuts for Formula 2. He used to really like the Angel Formula, but his preference now is clearly for Formula 2.
The green thing may be particular to Regals, as mine will also demolish any live Caulerpa that I put in for him (which I grown in a separate tank now). However, he clearly likes sponge. I know this because I recently added a large (about the size of an orange) Red Ball Sponge (Cliona sp? - not sure on this) to add some colour to my tank and gain some experience with it. Not long after putting it in my tank, My Regal started browsing on it. He doesn't go at it, but clearly takes very small bites. While the bites are barely noticeable, quite often you can observe him doing this as he takes quite an interest in it. I now liken it to a "salt lick" - albeit an expensive one! I also added a largish leather coral that had some yellow sponge attached to the rock it was attached to. My Regal cleaned these up in about a day (no idea what species - the amount was very small). I'm not too concerned about the sponge and if it gets eat over the year - so be it. I don't think this will be likely as he still gorges himself when I feed him the ON frozen foods (which I do 2x day - with a 1 day break every week or so). It was a calculated risk, but as my Regal reins supreme I'm happy to keep him happy.
