Grey skills do matter in my opinion. It's just that if you have an MC with high crossing it's not that much useful for him as an MC will seldom be in a position for crossing. Saying this sometimes MC do drift down the flank and cross.
The whole point of having a mutant is that we can keep our overall team quality low by focusing on the stats which a player will use more frequently in a game. This helps in getting easier draw. But now you are at an advantage because your players are in reality or 5 or 6 star but have necessary stats of an 8, 9 star player.
For DC speed is a grey skill and there must be a mutant DC with 1 percent speed. But that doesn't mean that they can't run. When they weren't mutant they would have had decent speed stat anf it's not that players lose their skill in top eleven it's just that there skills gets adjusted to their level. So a DC or ST with 1 percent stat doesn't mean 1 percent in reality.