Gives steel types that are already somewhat broken a different type of Pokemon that they can counter.
Attempts to tackle the underuse of Poison in completely the wrong way, as the reason there are so few Poison Pokemon in OU is because most Pokemon that are Poison are shit (as opposed to the type itself being shit, most Pokemon with a part poison typing benefit from it).
It also will stagnate the metagame further. Dragon offense is currently a valid playstyle that has to be countered in its own specific way. Including a Fairy type kills off this entire playstyle because you can just counter it in the same, bland way you counter most other playstyles (repeatedly sending out Pokemon that wall the attacker completely).
This is a bad thing, because it limits variety and means that even more games will become a series of continual switches.
Dragon as a type isn't overpowered, it's just that most of the Pokemon with it have very desirable stats and movepools. Few people suggest Water is an overpowered type, yet it has the most Pokemon in OU this gen and was still dominant in generations before rain was introduced.
I don't ever even play with DragMag teams etc. because I personally think they're retarded. I do, however, like seeing a team matchup that forces me to actually think in a different way strategically speaking. Dragon offense isn't particularly overpowered either, it's just that newbies tend to struggle against it greatly; look how few Dragon-centric teams there are used by players in the top 20.
