I have been running 30 miles a week too, having similar questions, and getting similar confusing answers.

From my readings it seems to me, it depends. What is your goal? Simply to run faster, longer - interval training, hill work the best way.
To get bigger muscles - you need something more resistance than a 10 % incline.

Having said this, I am proud to say that I have killer legs from simply running, without ever using any gym equipment. But for a woman "killer legs" are different, than for a man:-)
I heard a problem with gym training: it is hard to find the correct balance. If any of the "runner" muslces are trained musch more stronger than the others, it can casue injury during running. But you can prevent this with a good trainer.

I prefer a mixed approach. Because running is hard on my very sensitive lower back, and because I want killer butts to the killer legs, I have regular butt/lower back work-out.

I am not an expert, only shared my own experience (maybe I should post a pic of my divine legs:-), and my own readings about this subject.