It`s a big difference if you use Blender for simple, small models or if you design your whole environments with it using certain 64k Poly-Meshes with many textures. There`re many important things you have to worry about while exporting and importing (level structure, object placements, texture tree..). FBX eport can`t handle that task at all. And then there`s the workflow. With the Gamestudio exporter I can export the fully working level within two minutes. With the FBX export (if it would work at all), I`d need to convert all the stuff, check the textures, .. and I`d need about 30 minutes with the crappy MED in the crappy workflow.
FBX export might work for simple models with one mesh and one texture. But it`s unusable for serious tasks.