I use slicks including on trails. If it's mountain biking, mud, that is different, but on crushed stone, bare earth, sand, slicks work OK. Riding slicks on pavement is worth it for the great roll. With the right gear flats can be fixed in the time it takes to pump gas in a car. So while one doesn't want a lot of flats, one every few thousand miles is reasonable, one shouldn't sacrifice roll and comfort in order to avoid the occasional flat. There are conditions where flats are overly common and special measures need to be taken.

Here is an interesting article about tire pressure. Synopsis is that while rolling resistance might be lower with high pressure, there are diminishing returns. In addition, scientific studies show that road shock costs out at the felt level of the vibration. So if it feels rough riding hard tires, it isn't merely a discomfort, but a commensurate energy expenditure, so less pressure can be more efficiency.

http://janheine.wordpress.com/2010/1...-and-pressure/