There are so many theories in the diamond world, and everything relates to one another...operator, machine quality/rigidity, available speeds, wet or dry, material, removal rates etc.
Metal Bond diamond wheels contain metal fillers like cobalt, bronze, copper, tin and more. Those fillers create a lot of heat, which is not good for the diamond itself. They are meant to withstand the high abrasiveness of glass, stone and ceramics. Wet grinding also helps to flush away the material being ground and the diamond/bond fragments that are being ground off the wheel. Not grinding wet can cause the wheel to load up and create even more heat. Health-wise grinding wet will allow most of the contaminates coming from the material being ground and the wheel itself to be flushed away and not be dispersed into the air you breathe.
The wheel shown in your photo is a diamond electroplated wheel which is made up of one layer of a certain diamond grit size coated with a nickle plating. Electroplated can indeed be used dry as there is only the nickel plating involved. Of course we always recommend wet to increase the life of the diamond (heat is a diamond's worst enemy...it tends to break down prematurely). The through holes in your wheel can also help to cool down the application slightly.
By the way, once you notice your electroplated wheel wearing down, turn it around and you might be able to get a bit more life out of it.
Glass is a recommended material for electroplated, metal bond and resin bond diamond wheels, as is all other non-ferrous yet hard or abrasive materials including carbide and stone and plastics. Diamond can grind steel, but the chemical properties associated with grinding iron will eventually prematurely break down the diamond to its original state of carbon. At this point it will wear more quickly had you not used it on steel. CBN is the synthetic crystal that is recommended for grinding iron based materials with a minimum Rockwell hardness of 45c (in most cases).
Again its all theory based...if you are just touching up the odd steel here and there you probably won't see much wear. If you were continuously grinding steel you would most likely see a rapid wear rate.
Metal bond("copper bonded") wheels will typically grind : glass, ceramics, porcelain, stone, carbide and perhaps gem stones.