So I was watching the Shadows Fall live dvd the other day and the lead guitarist said "Don't get a seven string guitar kids, this is where you'll end up. Learn your scales!" and I want to know why you wouldn't need scales if you played a seven string. What's the difference?
He'd probably associate 7-string towards heavy metal strumming IMO. And that its easy to get stuck by that. But scales works just as same in 7 or 6 string and even other instruments. The only difference is that you have another low B extra string thats all, plus the neck is a little wider.
"Play with your ears" - Yngwie Malmsteen, Paul Gilbert
Thats what she said...
I don't see the Point in the 7 string guitars if I'm honest but then, I don't see the point in scales I use all solo places and use special out of key bends and that's how I got into limited music school kids with a very confusing guitar solo.
Do you're self a favour keep an open mind don't just do as you're told.
Probably cos if you have seven strings, you're drawn to play low-end powerchords. However, 7-strings can be used for solos, too. They have a wider range so you can make longer runs.

See also: Rusty Cooley and Jeff Loomis.
Steve Vai was probably the one person who brought it back into popularity the most.