A strong gluteus maximus, one that used correctly is important. Besides being one of the biggest, muscles in the human body, it also is a stablizing muslce and functions to right the body after bending over. All those weight lifters you see? They are engaging their "butt", not their backs to lift weights. Likewise for all of us....in everyday life, lifting can cause serious back injury. This is needless, if people were to learn to properly use their butt muscles. Yes, these muscles should be properly strengthened.
I haven't read from my yoga manuals, a muscular butt is bad for the body??? I think the author you are referring, is talking about "abductor" muscles, which when over worked and strengthened, can pull your body out of alignment.
Of course, all this doesn't matter, if you have a weak core...
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"All things are our relatives; what we do to everything, we do to ourselves. All is really ONE."
Lakota leader Black Elk