I guess I don't really understand that. Obviously I wouldn't want DT becoming a death metal band (although it might actually be awesome), but having one section of well placed harsh vocals doesn't seem unreasonable to me.
Although I don't expect them to ever do it.
Well, I just feel like if they had done it, it would've been like crossing into a whole new territory, which feels like it would result in them (namely MP) wanting to use more and more of it, until half the songs involved them, diminishing JLB's role in the band more and more.
I could be wrong, but heck, look at what happened when MP started singing in the band. It started with just singing harmonies, and then we'd hear more and more of him, until we had him singing whole parts. And sure, what we had on SC wasn't bad, I actually didn't mind some of his parts, and even a few of them on BC&SL weren't completely awful. But still, it wasn't particularly a direction I liked them going towards.
In terms of vocals, I'd say MP leaving was the biggest blessing of all.