grey mods being trash is not a myth.. the fact that it is grey means that you can ONLY get 1... lets call it layer of the secondarys. if it was a gold then each of the secondaries can go up more and more. so if you are looking for speed secondaries which everyone is.. you cant get more than like 5 speed from a grey mod as opposed to like 10+ speed mods which are extremely desirable.
A grey mod that is a speed primary arrow or a crit damage triangle then sure it will be useful but a random square grey mod or w/e is garbage and you really shouldnt be wasting 100k+ credits to see if you got lucky with a small amount of speed.