Hi Ghengis,
A great topic and a subject which could use some work. No doubt you are on to is - which is why you asked the question! Here's my 2p worth.
I trained in the RAF where we learned the checklists by heart. We were professional full time trainees and the time and resources were on our side. And as we were ultimately training to fly single seat fighters, it was the best policy as learning them was the only option for that situation.
Later, in multi-engine training (not having made the single seat fighter path!) we used challenge and response checklists. They seemed slow and inefficient to us at the time, but with practice and knowledge, they became more expedient and relevant.
First type after training, (multi engine) we did the first part of the checks by memory (the Phase One checks). the Phase Twos were challenge and response largely confirming actions from the Phase Ones. This was a great system and very expedient. Now, modern airliners use an adaptation of this where the much reduced checklists are confirmation of pre checklist flows.
When I started GA flying, I was surprised at the way checklists were used. They were overly long, complex, inefficient and sometimes irrelevant. I got the impression that some were written to train pilots to operate their light aircraft they way the authors assumed airliners were operated. While instructing on PA28s, I often sat there watching the engine turn while the student struggled through the checks. I would frequently point out that they could become more familiar with the checks before the flight and use the time they were paying for in the air and not on the ground. But all the time, I was thinking that the checklists could be changed.
The only GA flying I do now is in a small aerobatic aircraft. Before my first flight on the type, I asked the instructor about a checklist and he responded that there wasn't one. "What sort of checklist do you need for an aircraft with fixed gear and no flaps?" was his reply. So we strapped it on, started it up and went flying using airmanship and common sense.
One point someone has made is that the POH is part of the certification process and that the checklist within is technically mandatory, no matter how bad it may be. You can add items, but not remove them.