You can never have too many gauges.
But seriously, the reason for gauges is to identify problems or preclude things from becoming problems.
The EGT gauge (pyrometer) will keep you from melting pistons if you have enhanced your engine to where it's cappable of doing that. I don't think most factory stock engines/ PCM programming are capable of getting that hot. The second reason for the EGT gauge is to let you know when your engine has cooled off enough to be safely shutdown without harming the turbo.
The boost gauge will help you identify other problems like blown or leaking connector hoses or a turbo that is on it's last legs.
I have two other gauges in addition to the two above, a fuel pressure gauge and a vacume gauge to monitor the performance of that system. Not only does my HVAC run on the vacume, but also my exhaust brake.
You can mount up to 3 gauges right on the A-pilar.