Simply put: It will NOT increase bonus exp or MF/GF (Thats what Monster POWER is for) it will just simply multiple the density of monsters in every area, based upon the selection of Density you've selected from the drop down menu.
Right now would be Monster Density 0. All the way up to Monster Density 10, which would basically pack every area that has monsters, with even more monsters.
I know some of you are like "well that will increase the load on the game or the servers" well technically no. It would just increase the performance on YOUR computer. If your computer cannot handle the extreme density of MD10, then obviously you don't play on MD10.
For those of you who are hardcore enough and doing like 400k unbuffed DPS, you can run MP10 and MD10, increasing your loot by up to 10x, and making farming even more efficient.