Yes, barends work great for adding additional leverage when climbing, allowing more weight transfer to the rear wheel, but additionally they give several different hand positions allowing you to reduce overall hand fatigue. I believe this is why you see so many barends on department store bikes or bikes ridden primarily on the road. Also, off-road they tend to act as grabby hands in tight, wooded conditions, which is why I believe riders who ride in those conditions remove them.

Each of my mountain bikes that get a lot of road use have barends, including my commuter and urban assault bikes (both of which often see 40-60 mile days). My 29er SS however seldom gets many road miles and is often used in tight wooded sections, and it has no barends. If I still rode CA fireroads and arroyo single tracks it would get barends too.

As for the average neighborhood cruisers having their barend pointed straight up or even backwards, think about how many "road bikes" you see with their drop bar upside down. It looks stupid as hell to you and I, but its their bike and they want a more upright seating position and barends are the simplest and cheapest way to achieve that. Whatever floats their boat.....