This may not happen, but I have a desire to put a aux switch in so I can turn the fan on at my will and that isn't really worth it if I don't know how hot it is.
A manual fan switch remains totally redundant to a functioning thermal switch, IMHO. Further, considerable temperature difference may exist between the coolant in the cylinder head (where the temperature gauge sensor is located) and at the bottom of the radiator (where the thermal switch rests).
At the end of the day, the KLR650 engine is dependent upon radiator air flow (including any fan action) and coolant circulation for cooling; getting a "head start" on the thermal switch with a manual switch is at best, a limited short-term cooling enhancement--the steady-state thermodynamics don't change with a manually-activated fan.
A manual switch may offer the ILLUSION of control, but . . . only an illusion, seems to me.
More useful, IMHO, is a light bulb indicating fan activation; connect the bulb between the BLUE fan lead and frame ground. Such an indicator tells, end-to-end, whether thermal switch and fan relay are operational.
I have both indicator light and fan switch on my KLR250; consider indicator light useful, manual switch--not so much. YMMV!