If I'm traveling by air, I usually carry my passport as backup ID--especially after somehow losing my license once between the car and the counter at the airport. My Global Entry card is probably sufficient backup to be honest but I also have backup credit cards etc. anyway.
Global entry card is real ID compliant and a suitable backup. If you lose it, there’s not much somebody can do with it. If you lose your passport though, that’s just a more stressful thing to have to resolve
