It really does come down to conditions-- and those should be expressed in miles, never in days, weeks, or months. The popular tactic of many of the BF faithful is to claim, "Oh, i have/haven't had this happen in XXX <time frame here>," which is effectively meaningless. I ride a decent amount-- averaging 900 miles a month. My wife gets in between 240-300 a month, depending on weather and work schedule. So her getting ~2 flats per month without liners is inconvenient and annoying. That flatting rate for me would spell an end to cycling for me.

Since installing the Rhinodillos in her tires (Clement Strada LGGs) she hasn't had a flat in over 1800 miles. And I personally think this is a significantly better choice than HD "urban" tires. She's running 120tpi Stradas which are ~240g each, Rhinodillos are ~60g each, so the combo is ~300g-- compare this to say a Marathon Plus, at 595g for a 700x25. So while liners may remain an imperfect solution, a pair of good high thread-count tires + liners weighs about as much as just one touring tire.