Ben,
I fly across oceans regularly to play 4,5's and 6's(as ranked by someone else).
It would be a sad day indeed(for me) if I ever put a preassigned number (by someone else no less) as the reason I wouldn't travel to a course.
Many of these courses are my favorites anywhere in the world and I return as often as possible.There are almost always elements of a course experience (individual great holes,uniqueness,nostalgia, topography,scenery,rawness/remoteness,pub/town, companionship/nightlife, competition) that I would rate a 9-10.
Dunfanaghy would be a perfect example of this on a course that many would rate at 4.Rented a house on the 6th hole this last September for the second year in a row.
I do make it a point to travel with 2-3 invitees that rate no lower than 8 as people however, so as long as we play a 3 or higher we do OK