Plenty? Therein lies my point. If we're killing the beasties, we ought'a get be allowed to know their *proclivities.
*aka their strengths and weaknesses, on inspection/examination.
While I would never say no to more MM's. At one point Severlin said they wouldn't be making anymore MM's. There was a 5 year gap between 7 and 8. Now we have gotten 2 in a 2 year period with the release of 9 with Vecna. Personally I am content if SSG produces a new MM every 2 years.
They are a good source of XP. My personal leveling guide has MM hits clocked at 392k in xp in a 1-20. (using a 30%, 10% vip, 5% ship, 5% gift)
As it is one should start to figure out what is need to take on a monster well before the strength and weakness of the monster are unlocked. So one would have to put in some time anyway killing the things.