From what I've read so far, Muscle Monks/Priests are literal walls. And have very few ways to attack back. While the other classes are like normal tanks we have in our MMORPGs that have a few attacking skills, their system is different. If you want to attack, you need to have the skills of whatever weapon you're using. As MM/MP only use shields, they don't have any options besides the typical shield bash attack. You don't need a wand to cast [Heal], so they can use a larger shield to tank better.