Actually having bred and taking in "rescue" rabbits for a while, I can vouch that the cage doesnt have to be visibly dirty for bumble foot. See walking on the wire can after awhile damage their feet, then since waste is falling thru them wires bacteria is in contact with the open sores. Even if they are kept clean unless you are bleaching the wires constantly (which would be more work than having a solid floor) and washing their feet there is a chance of getting it.
Also the multi level cages have a solid flooring so they can get off the wire when thier feet begin to hurt.
Ive seen some pretty nasty cases in rabbits and for me its easier to avoid the problem than dealing with it. Of course rabbits are alot different than rats and Im sure you would never let your rats get half as bad as the animals Ive seen.