It takes one keystroke and two mouse clicks to completely change specs, and it can be done anytime and anywhere as long as you are not in combat. People who realize that and take advantage of it, are going to find a LOT more varied play with their classes then those who stick with a single spec. So for example, if you are out on your own taking on normal landscape mobs, you won't need any survivability skills. Run into a signature, or elite, or some other difficult content, and a couple of seconds later you've switched to a spec that can handle it. So rather then cluttering up you toolbar with every possible skill you might ever want under any possible circumstance, you simply switch specs to match what you are doing at the time.