Devil's advocate: Are these bands good for metal?

Nu-metal bands, and especially metalcore bands, are good for getting people into metal. I don't know about cock rock (Led Zeppelin, Motley Crue), though, since those bands aren't nearly as heavy as true metal, whereas some metalcore and nu-metal is almost as heavy as death metal, albeit crappy music.
When i was 12-14/15 i listened to Nu-metal, just because itwas the only thing i could get my hands on. I always was looking for the most complex things out there, but all I could find is Nu-Metal, since MTV and most the internet had it, but I had a friend that could tell I wanted something different. I think if the "extreme" metal forms were more accessible, that Nu-Metal and most Metalcore would be obsolete because its usually a phase someone goes through because everyone realizes Nu-Metal and Metalcore are boring....
Hard rock - especially 70. classic - is great music, so of course it can't be bad...

Situation is much more complicated when we think about nu-metal. I can imagine both the situation when somebody hears numetal song,gets interested in heavy music, discovers other kinds of metal etc. and the situation when somebody hears some nu-metal song and says 'This is horrible, I don't like metal', ant gets prejudiced to the whole genre because he/she had a misfortune to hear something really bad at the start.
I started listening to Metalcore like Killswitch Engage, Atreyu, etc. for about 3 weeks and after that was into Melodic Death Metal, not too long after I was able to stomach Death Metal like Morbid Angel and started getting into some Black Metal. I hate Metalcore stuff now but I guess it was instrumental into helping me get into Metal.

Personally I think a lot of people who get exposed to Nu-Metal and Metalcore will probably stay liking those genres. Nu-Metal is more of a mainstream thing, if those kids wern't listening to SlipKnot they'd probably be listening to My Chemical Romance or Jay-Z. Metalcore on the other hand is more underground genre but it promotes the same "scene" principles that are closely related to Emo-rock and Nu-Metal.