In my considered opinion AC 120-91 should be regulatory and it should be updated to provide RNAV and even tight RNP OEI flight path options. But, this is a classic case of the "tombstone factor" at the FAA when something as critical as payload politics is involved.
Payload politics for sure...
In working with these performance profiles, it really is very difficult given the number of variants, engines, configurations, bleeds, etc that is would be difficult for the FAA to regulate how this is accomplished.
For the most part, when the carrier is shown the EO performance, it is usually a bit of a shock with the weight limits or temperature limits. Many regulators then default to EO as 'emergency operations' and let it go at that, again, thank goodness for the reliability of todays engines.
For the most part, the people back at Ops are using BCOP or something similar, to load the aircraft, with assumptions for the destination, as well as departure.
The pilot should do a quick check on approach to make sure the assumptions are close to real time conditions, namely temperature and weight, and adjust if necessary.