you just answered your own question. stability. flimsy stuff will sag, allowing your substrate to enter the water column, creating a bog. now, this isn't an issue, if you have bog plants, but without bog plants, nasty bacteria can grow in a wet substrate, and possibly get your frogs sick. egg crate also has really nice, big holes, allowing for water to drain from the substrate freely. i could probably go on, but those are the main points.