I'm not likely to do many more roofs,
but the reason I like it (so far) is
when I see guys cutting one side
all the way down they aren't able to
dog that top corner.
I'd say more than 75% of all the valley
leaks I've seen are from water catching
on that top point, and traveling back
from whatever the underlayment is.
It's super easy to nip that top corner.....
Snapping a line and getting a strait valley is also very easy. A well trained monkey or a untrained human can cut a strait well placed chalked line down a shingled valley on a warm afternoon.
I put down Ice & Water in every valley regardless as a fail safe, as it's recommended by any manufacture. But I won't cut a corner because it's there.