The lid on a tip in duster is outside of the cage, so the duster has to be tipped out or the animal will have 24/7 access to the dust.
Personally I think tip in dusters are nice, what I do not like is that most cages that have them use them as the door to the cage, which is far to small for my liking. I like a full opening front personally, but in a larger pet cage you could have a tip in and a door and I think it would be a nice setup