clutter and fragility... it is forever the same conflict whenever there is anything added to a bike.
FWIW with dynamo wiring I think it is often neatness - which favours hidden wiring- vs repairability, which doesn't.
On a bike with a rear disc brake (hose or full outer) the dynamo cable can be routed alongside the brake stuff, as far as the rear hub. One neat method of securing the dynamo cable to a mudguard stay is to use electrical heat shrink insulation. Black mudguard stays, anyone...? I have also considered making one mudguard stay from a thin-walled tube and routing the wire up that instead, but thus far it has been 'one faff beyond' and I have not pursued it.
On a bike without a rear carrier fitted, with rim brakes, mounting the dynamo light high up on the seat stays allows the cable to be run alongside the brake cable, leaving very little exposed wire to worry about.
[edit; I see that others use the heat shrink approach too.]
cheers