I have this vac switch as a standard. And am very happy with it.
In high you have to press the switch to get 4WD as we all know.
In low 4WD is 'on' automatically because, when moving the lever to low, a switch on the transfer does the same as the 4H-switch does: engage 4WD.
I disconnected this switch on the transfer and now I have push the 4H every time I want 4WD, even in LOW.
Big advantage is that tight cornering is easier and during manoeuvring in tight spots (especially with a trailer) one can use LOW making it less harsh for the clutch.
Keep in mind though, that, if you are using LOW in 2WD-mode, ALL the power is delivered to the rear drivetrain only.
Another thing to keep in mind is to not forget to press the button when really needing 4WD in low