Direct arm work is just for fun ? So if someone purchases some free weights and does curls their biceps won't get bigger after 1 or 2 months?
direct arm work is a necessity. There's a reason every single successful bodybuilder/physique competitor trains their arms. Newbs, however, don't need more than a couple of sets thrown in at the end of a workout. There's a tendency for beginners to do waaaaaay to much volume for their arms, and it becomes counter-productive.
As for someone buying some dumbells and just doing curls, well yeah, their arms would get bigger, however it neglects the big picture. Having decent looking arms is more than about just developing the biceps. If you want a decent looking set of arms you need big biceps, triceps, shoulders and traps. Another example would be someone wanting to develop their shoulders, so they buy some dumbells and just do lateral raises. While you do need a well developed lateral deltoid to have impressive shoulders, you also need a good upper chest, rear delts and traps.
So yeah. In short, arm work is good, but there's no need for newbs to do more than 2 or 3 sets of it a session