Is autofocus always important?

I think that getting a Nikkor AF 50mm lens would be better than getting an AF-S one.
My camera doesn't have an autofocus motor.

Why or when is autofocus important?

You are really thinking that an AF lens will cost you less than the AF-S lens.AF-S lenses auto-focus faster on cameras with built-in auto-focus motors.

When I shoot action and sports, auto focus is a must.

If you are competent using the electronic rangefinder in your cameras viewfinder, then using an AF lens is fine and when you eventually buy another camera that is NOT an entry level camera, it will auto-focus on it.

Most of my lenses are AF-S, only because they are newer lenses. Eventually all Nikkor auto-focus lenses will be AF-S