Just my preference I guess. If I am not losing any power by having a pigtail, I would rather have the convenience of a plug. Just puzzled why Rivian is so adamant about telling customers that it can't/should not be done. You are just disconnecting the breaker wires and replacing them with correct pigtail wires leading to a male NEMA plug, charger will/does operate the same.
It's not the same power, though. A typical plug-in installation would be limited to 40A charging, compared to 48A for hardwired. Many jurisdictions also require GFCI on an outlet, which is a known problem for EVSEs; hardwired installation generally avoids that issue. Rivian wants the best user experience and fewest support issues. Hardwired accomplishes that.