Why? Well, let's say you only kill Moonstalkers. Every time you kill one, the server decides to spawn another mob. It looks at its spawn table, sees Moonstalkers and Foreststriders, and randomly picks one to spawn. If it's a Moonstalker, eventually you'll find and kill it. Fine. But if it's a Foreststrider that's spawned, you won't kill it, because you're only looking for Moonstalkers. Eventually, the zone will become entirely Foreststriders (unless you're unreasonably lucky), and you'll be scratching your head looking all over for your quest mobs.
Not to mention, you get more XP per hour killing everything you come across (if you're below 60, of course). Anyway, the good people over at Kill Ten Rats lay it out very well, so go forth and be edified.