There also was an accident that showed the value of EPR over N1. In 2004, a F70 of Austrian
suffered a forced landing on a field not far from EDDM (Report) due to a rather peculiar double engine failure: in severe icing, the constant barrage of ice shed by the fan and thrown at the protective strips just behind the fan caused them to come loose. These strips were of a size and material that did not allow them to depart the engine down the normal airpath; they accumulated in front of the fan stator vanes and blocked the air path.
So the engines were both spinning as they were expected to. N1, N2 and EGT was indicated and within normal ranges, but what was lacking was EPR and consequently the expected thrust.
Both approaches have their benefits and downsides. It is possible for both indications to show normal values with not nearly enough thrust being developed, and to show this behaviour on all engines installed on an aircraft at once.
EPR is simply one more measurement showing the state of an engine. Also if it is not used to set power ratings, I think it is a valuable information to have in order to get a rounded picture and to see that everything is in order.