I haven't installed a 6 inch light in years, but I've replaced a few with 4 inch. Anyway, I think the whole thing is an issue of fashion over function, so it isn't surprising that the fads are different.
We have to install a vapor barrier over the lights. Think about cutting a hole over the kitchen sink, putting your hand up and touching the roof above the hole. Now imagine crawling out to the hole from above, pushing insulation and spanning the rafters with your elbows and knees. That's why my family wears head lamps when we do dishes. I'll do that work for money but not for love.
