I'll add a rec for Mr Tuffy's (and there are other brands of liners as well, Tuffy is what I have experience with). Maybe some people feel a difference due to additional circumferential weight, but most probably don't (and besides, reinforced tires will have that weight as well). The plus of using liners is they're pretty cheap ($15/pr for Tuffy?), and you can put them into cheap tires and also put them into replacement tires. That way your long term cost is not higher-priced tires all the time. If you are a higher-end rider and are going to spend a lot for higher-end tires anyways, then perhaps you would be better off locating your flat protection in the tire, rather in the liner.

Didn't read the whole thread closely, but another concept is to go tubeless with Stans' or some other sealant. There's a pretty mindblowing video on the Stans webpage of running a mtb through a torture chamber consisting of a groove in a board with nails poking in at all angles. He rides the bike through that grove over and over and over, every nail fully puncturing ever time, and loses barely any air pressure throughout it.