The tyre I'm using as a liner is a 23, the outer is a 25. With the bead cut off, it sits inside and prevents any sharps getting in, but allows me to wear the tyre right down on the back without getting lots of punctures because of it going thin. In fact, I've not had any punctures since I started doing this. Yes, it's 50 % saving money- which it does very well. But it's also saving me time on the saddle which is better than fixing flats in the wet and cold. Roads get horrible in winter, all the flints wash onto them, and punctures are much more likely on wet roads- wet flints are lubricated on their way through your tyre. Our roads are always wet this time of year.

I do this with the back tyre, as that's the one that wears down, and I use the newest tyre on the front. Yes, it worsens grip, and probably makes the rolling resistance quite extreme. But it's winter! My mudguards and 69" fixed gear slow me down just as much, and I don't honestly care. I just want to get the miles in. It's not getting me dropped from my training rides, and I've not yet fallen this winter- can't really push it on corners with fixed anyway, and if the tyre squirms a bit in the wet I don't really care. Gatorskins aren't exactly a tyre you buy for cornering grip or fast rolling, are they?

Yes I remember the tyre liners from my mountain bike when I was a kid. They were utterly fail, for all the reasons you mention. And glueing them doesn't help anyway, they soon come loose. The doubled tyre method has none of these defects though. It seems to be the best way of getting your moneys worth while training. Why would I get new tyres so often, and still get punctures, when this method allows me to get 3X as long out of them and never get punctures? I can save that money for nice fresh tubs to use when rolling resistance and cornering grip actually matters. Train hard, race easy, ya bunch of BF pansies.