The thing is, there is nothing inherently necessary about having different mechanics. There are also inevitable problems with having different mechanics however as in when you use different mechanics, inevitably some mechanics will work better than others. This leads to unintended imbalances in the game. Cohesive design is aided by making things work off of the same skeleton. I'm not saying that different mechanics is badwrongfun, I'm just saying that the game as a whole runs more smoothly without them, and that they are unnecessary.