Rule seems to be if you have to go to a desk with an e-ticket then they want to see ID for each traveller. If you are travelling alone, they regards the credit card which paid for the ticket as ID. If you check in on line or use the machines, they don't, unless you want to check in hold baggage when they do.
I remember when FlyBE first started to ask me for photo ID they gave security as the reason. In reality IMHO they just wanted to stamp out the practice of people selling on their cheap tickets when they couldn't travel.