I have to agree with Mike Mccue. I've never heard doubled vocals that didn't sound like doubled vocals. I've been listening to music for most of my 40 years. And I listen to all new stufff and the classics. The doubled John Lennon vocals, some of it I think sounds crappy. But the other guy, Paul was it, he had a more "musical" voice.
But some of my favorite indie artist do major doubling. Listen to Elliot Smith, he's all doubled, all the time. One of THE greatest indie artists ever.
And Bon Iver, some of his albums he's like quadrupled sounds like. Mass doubling and effects. But the thing is, I've seen him live on youtube and he sounds good without doubling.
Sometimes people just use the voice more as another instument more than the "voice", so adding effects is just like effecting your guitar. a la pink floyd.
If doubling makes it sound better, than that's all that counts.