@The-Atog-Lord Magus of the Moat definitely has some advantages over Moat – and I really like it in Brian's list – but not enough that you see more Moats than Maguses in general. For the purposes of this thread, I think it's true that "adding legs doesn't ALWAYS make an effect better."
I think the differences are largely explored in vintage. Attacking and blocking is awesome, but it's easier to kill creatures than it is to kill artifacts (usually, depends on the matchup). Blood Moon/Magus of the Moon is often a tossup, I think.
As for this card, I think you would still rather have Crucible in a matchup like Landstill, where you really want Crucible to stick around, and they have lots of creature removal ... but the marginal advantage of a body against decks where Crucible isn't as key (e.g. Mentor), means that the card is more maindeckable than Crucible is. Tutorability by Green Sun's Zenith or other cards is another dynamic that isn't true about Magus of the Moon/Moat, so that's pretty interesting, too