Matoran (well, toa and turaga) have been known to eat with mouths, haven't they? They technically can, but they typically don't unless it's medicine or they need to absorb a specific nutrient rather than pure energy. Fanon is that they think it's utterly revolting and bestial, but I don't know if there was ever any canon statement on that. It has been stated in Biosector in the Skakdi info page that those beings consume and digest their food which the other MU inhabitance see as revolting and bestial. I'm pretty sure that another source states that as well. (not sure if it was BIONICLE Encyclopedia, or BIONICLE World, but I'm certain I've read it somewhere other than Biosector)