I agree with you on the surface. However the time to do that is in cruise flight when you have the time to examine the approach plate.
I have with me a small booklet with all the info already published, so there is no need for long calculations. I simply turn to the appropriate FPA page and read off the distance and height required. Takes about 20 seconds.
I fail to understand why airlines are so reluctant to capitalize on new equipment that can make flying simpler and safer. I am sure that the development of the FPA feature on the A320 family aircraft must have cost a wopping lot of money yet I find most pilots unwilling to use it.
The manual published by Airbus (two volumes) on CFIT accidents points to step-down non-precision approaches as being the biggest cause of fatal accidents and goes on to indicate that a single constant FPA would in almost all cases have prevented the ground contact.
I have always increased the FPA to 3 degrees (5.2% Grad) on any approach that is published at less than 3. As mentioned above the recommended safe FPA for JET aircraft is from 3 degrees to 3.7 degrees yet there are many approaches published at less than 3 degrees. (The chart makers are only concerned with obstacle clearance).
It is necessary to have the FPA charts on hand for the 3 to 3.7 degree slopes and to use them (with raw data backup and dme vs altuitude crosscheck) until one becomes comfortable with the procedure. It's a lot easier than you might think, just needs pratice like anything else. A lot safer too.
We should use EVERY bit of information in the cockpit to our advantage and the FPA feature is no exception. Imho it's the best invention pilots have had for many years, so lets use it. I used the Lotus 123 spreadsheet on my computer to do the charts and if I can do it, anyone can.
Thermostat.