Way back when I was first taught to use WordPress, I was told to use /%postname%/ and I have been using it ever since.

If I had included the category, then my URLs would be longer and I for one don't want longer URLs.

That's all depends on the situation. Usually I would say /%postname%/ but there are situations where category would be better. Let's say I'm doing a small review site, all centered around 1 product...

The product name is xyz. I would make the category for all the posts xyz and make the permalink include category and then the postname would probably include xyz review.