Multi bios is again probably just for developers (switch between a dev bios to a normal bios)
Now, to say that you only need 512k if you know what you're using it for is not REALLY true... MS or other developers might figure out a way to prevent people from using mod chips with regular backups (not just online). I'm sure they already have figured out a way, as it is not hard to find the checksum of the bios on the machine....but, anyways...if they decide to prevent CURRENT hacked bioses to play games a NEW bios may have to come out. If this is the case, one might HAVE to use 512k - this is unlikely but team X-ecuter did say that no bios here on out (developed by them) will be smaller than 512k. Evox might come out with a new version, if needed, that's 256k...but, who knows what will be possible.
This is all of course speculation....but, one day new released xbox games might not play on an xbox with a cracked 256k bios. If this is the case however, then you would simply put your matrix aside and buy a new mod.