I think kennaz's first post if you read it again, is perfectly fine. He mentioned its 'just a thought' and it doesn't really bother him that much.
I'm sure it doesn't really bother any of us that much, but half the forums around the world block guests from certain stuff, so he's not the only one who thinks its a smart way to operate. Bigger member lists are your no. one goal as a forum administrator or owner. You will also find that forums that do block you, tend to have a much greater wealth of good info because among those guests could be some guys doing some really trick stuff with motors etc. who just are too lazy to sign up or consider themselves 'above the forum'. Some just don't want to share their secrets. Once they are forced to sign up to get at some info they want, they may just start to post cause they know they can now. They all get enticed eventually if a thread gets the better of them. Some people just need a shov...
IMO you will find that free for all forums will be full of forum junky members, but membership requiring forums will have a lot more knowledgable members.
The best system IMO is to let you get at all the Threads but block the piccys. You gotta remember the guests are sucking bandwidth for nuttin!