Somewhat. Not so much skills as environment. Skills could drop some with disuse, but I would not make it extreme.
In a complete SG system, you grind mobs for one reason: Gain Skill and Loot.
In a complete SL system, you grind mobs to keep them from destroying your cities, skill and loot is besides the point.
In a SG/SL hybrid, you grind mobs to gain skill and loot as well as grind mobs to keep their numbers down so they don't burn your cities to the ground, invade your mines and make your roads unsafe. You just gradually do this over an area, making the inner part 'safe' for new people who want to start with a low character, and outer warzone areas that need highly skilled fighters to defend at all times (based on how many players are online). If the mobs are not constantly pushed back, the 'safe' circle shrinks as the mobs push in and easy resources become less available.