It's fairly simple: if you are designing an instrument approach, then you assume that the pilot cannot see (sometimes you have to state the obvious!) so all of the tracking criteria are based on the reliability and accuracy of the tracking aid the pilot will be using, and the buffers are applied to that.
If you are visual - then you can see obstacles in your path, and the only criteria is to avoid obstacles by 500'
unless you are landing - which (as you are conducting a visual approach) you are.
So - once visual (I mean REALLY visual, and able to remain visual to the runway, able to VISUALLY see obstacles such a church spires and the like) then you fly visually. Break off tracking using the aids, and fly your VISUAL approach. (i.e. look out of the bloody window!)
Most major airports have a formal requirement written somewhere (usually in the noise abatement pages) requiring visual approaches to be not below the glide path of the ILS - usually 3º.
Obvious to an old pilot, perhaps not so to the
Children of the Magenta Path ...
