AFR gauges (and all other gauges for that matter) are great for diagnosing problems or preventing a failure If you spot somthings wrong before anything gets damaged.
For things like fuel pressure, oil temp, coolant temp, EGT etc etc... you cant beat the defi system, I had my oil temp hit the alarm point at 130 deg C on track at silverstone while proving a point to a subaru TA car (that would not move over ha!) and if i did not have the defi alarm set up I would have NEVER noticed the oil temp was getting dangerously high! I would say that a well set up gauge monitoring system is well worth the out lay if it save your motor!