Infraction: 8. All accounts must have a valid WoW profile.
Message to User:
Your primary character is under level 10? How about you fix your profile.
You honestly do not need a huge amount of Armor Penetration just for MM to throw out decent numbers.
As for fights where MM is better, I consider any fight where you have to move a lot to be equal or better. Freya+3, Mimiron, Thorim, Yogg, hell, don't discount MM for Hodir and General either (Chimera is magic damage, arcane is magic damage, and the big magic crits = big piercing shots ticks). I stay MM full time and do quite well (in fact I switch to a MM spec with efficiency for vezax, and swap out tracking for range on Yogg).
Our WOL is down at the moment but to give you an idea I do around 6.9k DPS on an Ignis kill without pot (and my armor pen is only around 300 as well, although I do have 4 piece, skyforge and some other decent pieces).