The equipped creature still gets +3/+3
Result: Near infinite lifegain/life loss
For all its glitz and glamor, it's not an immensely powerful card, and it really only fits in decks that either care about counters or intend to gift your opponent creatures, as with Grismold, the Dreadsower and friends
This limit forced players to make a critical strategic choice during deck building: which single powerful effect was worth the slot
Analyze the Pollen Like Traverse the Ulvenwald, this can tutor land or creature effectively and rewards self-mill as a means to collect evidence
Playing around Magus of the Moons effect in a format without Fetchlands isnt always easy, but there are definitely benefits to keeping the card around, especially when you factor in a new addition to Historic that didnt earn a ban