They do have some limited success at deterring "script kiddies" and similar, however I think that this is far more difficult to do on Linux than Windows. Back when there were a few games around on Linux that used PunkBuster (America's Army, Enemy Territory), PB offered almost no real protection and, e.g., ET's mod developers decided to build in some anti cheat protection of their own into the mods which was superior to PB's. (Which, of course, also got bypassed eventually)