Actually, they aren't extremely rare. The gold cartridges signify version 1.0. If you have a gold OoT, then the Islamic symbol of a crescent moon with a star will show up everywhere in the Spirit Temple and on the Mirror Shield, the music of the Fire Temple will be the Shahadra, and it will have a few more glitches than other versions.
The gold MM isn't special. It's a game with relatively less obvious glitches than OoT (although there are many glitches), so they didn't fix a lot of them in later versions.

LoZ and AoL gold cartridges are just the earliest releases. They're more common than you think.