Hmm, didn't quite realize that there were (seriously) flat-resistant skinny tires too. So it makes sense to buy a flat-resistant tire rather than mess with the tire liner. Changing a flat is not fun if it's dark/cold/raining. Extra cost of flat-resistant tires can mean less cost for tubes (esp for those who don't like/trust patching).

OTOH for Sunday riders on cheaper recreational bikes, tire liners can be nice insurance against flats which they are not equipped to fix.