Security is not the ONLY reason for checking boarding passes at the aircraft door! There are many reasons an airline may decide to do it.
It's a simple way of making sure some idiot doesn't get on the wrong plane! Believe me this happens, and when it does it causes a whole load of hassle, for the passenger, the crew and the ground staff. The airline can get fined, (which is then passed on to the handling agent) the passenger gets sent back on the next flight and the Daily Mail gets hold of the story and has a field day... Someone boarding the wrong flight may not always be a security risk, but it IS a complete pain for all those who have to deal with it.
Not everyone travels frequently, and you would be amazed at the number of people that turn up at the wrong gate simply because they have seen "Malaga" on a screen and don't realise there may be more than one flight going to Malaga at a time. When you have an easyjet/ryanair style free for all boarding scrum, passengers can and do get through the gate checks and try to board the wrong aircraft. This in turn causes delays, which annoys passengers and can lead to penalties for the handling agent involved.
You then get the people who think they can try and sneak onto an earlier flight than the one they booked to avoid paying a change fee. I agree this is more a revenue protection issue, but in todays financial climate, airlines need to protect their revenue more than ever.
An airline also needs to know exactly who is and isn't on board to ensure only accompanied baggage is allowed to travel (this IS a security issue - remember Lockerbie?). And as I said before, if an aircraft were to crash, they would be expected to have an ACCURATE passenger manifest for the authorities to use to contact relatives. Passenger reconciliation to satisfy these two requirements is done at the boarding gate - checking again at the aircraft door is just an extra measure implemented by some airlines. As far as I am aware it is not required by EU law, hence the fact not all airlines do it.
If an airline wants to check boarding passes at the door, they are fully entitled to. I appreciate that the inconsistencies can be frustrating, but each airline has its own rules, and as a passenger booking a ticket you agree to comply with those rules.
Is it really that difficult to show a small piece of paper at the door? And does it really matter WHY you are being asked for it? You never know, it might even be so the crew can welcome you by name, then direct you to your seat to try and speed up boarding and ensure you depart on time!