Well if you do it to all of the creatures then basically what the game will do is max out your population with creatures picked from all over the world. So you will probably still is have many more creature types then would normally be available, but you still won't have access to all of the creature types from only a single biome. Think of it this way in that in normal DF in a tropical biome you might get 15 different creatures. These creatures would be your basic tropical animals (elephants, lions, tigers, etc.) and then depending on where your fort is you might get a small amount of temperate creatures as well. When you add ALL_MAIN to all of the creatures, it basically means that you will now get the maximum number of types in your biome (say like 20 for example), but these creature types will be chosen randomly from all of the ones across the world. So you then might end up with arctic foxes, musk oxen, bears, and geese for example, regardless of the fact that normally some of those only live in the cold and some live in the tropics.
This will greatly increase the effect of starting fortresses on multiple biomes, however. This is because in vanilla sure you might have 15 creatures from one tropical biome and 15 from another, but maybe 10-13 of those creatures are the same. With this edit you make it so that even if you only get 15 from each biome, they might only have 1-2 in common, giving you an increase of 10-13 more creature types over what you would have had in vanilla. (Note that all of these numbers are just made up on the spot, but you get the idea I'm trying to say.)