Much like most other epics, MM is very good in some specific areas, but not really well-rounded enough to warrant being a legendary. He's got a great kit, don't get me wrong, but I personally find I'm not using him anywhere at all. Considering putting him in a very specific arena team, but that's about it. In comparison, almost every one of my legendary champs is useful in multiple locations.

Since MM is an epic, this makes him even more valuable! Epic books are relatively easy to acquire. 

And I disagree about his versatility strongly, not sure where you are in progression kram but MM can be useful everywhere but CB imo?

Dungeons: Check (obv spider is best for MM)

Arena: Check (you are correct that needs a niche team, but is great with the right setup)

Hard DT: Check (I was between 110 & 120 and MM crucial for me on Scarab and the Mortu waves).  

To be clear, I'm not suggesting he isn't usable in those other places. My point was more that legendary champions generally have a "permanent" place on at least one place, while epic ones tend to be replaced (eventually) by a better legendary.

As an example, MM is almost completely supplanted by Sir Nicholas, who, with the exception of not having the AOE stun, has a generally far superior kit - and that's just the most direct replacement.

Sir Nick neither does AOE stun, nor has a perfect-uptime AOE shield, nor the fear thing... there are some similarities in that both are HP-based champions that combine damage and support, but that's about it. You wouldn't use Sir Nick in spider, the scarab king, etc. - or at least, it would be much more complicated than with MM. 

And he's not the only epic like that - the likes of Tayrel, Serris, Umbral Enchantress, etc. aren't replaceable by any legendary, either.