It's just a tool. Sometimes different tools are useful for mitigating risk, when one knows how to use said tool properly.So I've had a reef tank for 5 years... Been using the same plastic, low-tech, submersible salinity checker, that I'll bet most of you will say I'm careless for using.
However, Two cups of Red Sea coral pro in 4 gallons of Ro Di water has yielded the same measurements... Always. Hard to argue with consistency... And a successful tank.
So why should I have the high tech one, that needs calibrated, etc, etc... I'm open to the idea but hard to argue against positive results.
Redundancy can be a good thing.
If what you are doing is obviously working for you, then continue along.
I like my combo of refractometer and conductivity probe.
If I suspect one may be "off", the other is there to confirm.
And the refractometer can be used where the probe can not, due to the portability factor.