It's actually a smart idea...we install convenience gauges under the hood as well as those added as seen from the driver's seat. some gauges duplicate the parameters available from scan tools, ScanMasters, dataloggers and the like.
we install a gauge on the fuel rail as a convenient quick scan to identify a fpr or injector problem (if your fuel pump has a check valve) or WOT fuel pressure on the dyno (duplicating the pressure transmitter feeding data to the dyno's computer) just as an example.
easier to monitor while installed on car. sure a 4 inch gauge for bench work works great when performing exacting diagnostics and repair but a 1.5 inch or 2 inch gauge tee'd at the brake switch will provide convenience monitoring whenever desired to review system charge pressure, pump cycle, system static holding pressure, etc.
Convenience monitoring provides easy system observation to identify an issue before it become a problem. for example, proactively identifying a failing accumulator diaphragm rather than over operate the pump cycle rate risking burning the winding while unknowingly waiting for the brake light to illuminate. or repeatedly blowing fuses from pump frequent operation. getting more expensive to re-wind electric motors and Hydac accumulators are not always in stock.
being able to identify an issue before it becomes a more expensive problem is working smartly. it's the difference between "this is something I need to look at via preventative maintenance" vs "I have to park the car" reaction due to failure. yes, there is the possibility of system failure without notice.
proactive convenience monitoring of the brake system pressure can be accomplished with a simple gauge and a little fabrication.