Under current rules, capital hexes are 18km across, and the longest-ranged weapons (aside from missiles) are 56 hexes at extreme, if you use the variable range optional rule. 18*56 = 1008 km max range. So for most practical moons, there isn't nearly enough range to defend the planet with ballistic/energy weapons. Units capable of independent maneuvering(ASF, capital missiles in bearings-only mode, etc.) can guide themselves to a target, so they're not limited by that range cap, but any direct-fire weapons are. Even a moon like Phobos is not nearly close enough - it's got an orbital radius of over 9000 km, and even if you measure surface-to-surface (instead of center-to-center), it's still about 6000 km.
My impression is that the first batch of rules, back in the 80s, used much larger hexes - big enough that moons or maybe even planets could fit into a single hex. Under those rules, the ranges ought to be sufficient to use moons to protect planets. But under StratOps, no. That said, using capital weapons to protect the moon base itself would still be a thing.