[JasonD]

Dot 5 does not absorb water, so any moisture (condensation, possible if not a fully sealed system) can end up sinking to the lowest point (calipers). The water corrodes the calipers. Water also has very low boiling point at the worst possible place in the system (again, calipers), which can lead to sudden no brake condition because of water vapor, but the fluid in the reservoir looks pristine. Regular bleeding helps avoid this unlikely but possible scenario.

Conventional Dot 4 and below: any moisture mixes readily, leading to a gradually lowering boiling point and degradation of the fluid. The fluid in the reservoir looks bad too. More predictable and gradual compared to the silicon fluid situation. Regular flushing to remove the degraded, moisture containing fluid.