You're absolutely correct that there is no extra battery drain while the computer is off but there is an additional battery drain if it's on and you don't have any need for it to be on if transmitter is not present.Not a big deal for me as I have no reason to really turn AI off when I'm diving and I don't believe having AI on has any drain on battery while the computer is turned off.
This is not a very likely scenario but there are times when I do switch out regs and may not have a transmitter on my other set (just bought another transmitter off Ebay so that scenario will not play itself out anymore).