Take Relapse. Now, the main complaint was that the 'accents' was used too much.
How many times did he really alter his voice. I'd say:
-3AM
-Stay Wide Awake
-Bagpipes From Baghdad
-Old Times Sake
-We Made You
-1/3 of Hello
-2/3 of Same Song & Dance
That's 6 whole songs out of sixteen. It's really not that much.
Just interesting that it's brought up some much when it's on about a third of the album.
EDIT: I'm just talking verses. He uses accents on the My Mom and MBTG chorus'.