I've never used any tire liners, and I went for about 3 years with only 1 flat while out riding. I generally don't ride in the gutter or near the edge of the road where junk accumulates. Then a few weeks ago I took my road bike commuter to the auto parts store, rode a mere 600 yards on a wide shoulder before getting to the store, and when I came out BAM, flat rear tire.

A week or two before that, I was commuting to work on my heavy utility bike, which was already taking a LONG time due to strong headwinds. Then I decided to get on a shoulder before turning in a neighborhood about a mile from the office, a different way than I normally go. Another 1/4 mile and my rear 1.5" tire was flat, and I stupidly had nothing with me to fix it that day.

I'm sure some tires can handle it, but generally shoulder riding is the devil. If I were in a position where I felt I had to regularly ride on a road shoulder I would probably go with something like Marathon Supreme tires before using tire liners.