Curiously (like CUPS) avahi (aka Bonjour) is one of the better things that Apple have developed*. For most home users being able to find their local devices with somehostname.local rather than 192.168.3.14 makes perfect sense to me.
I agree in principle (of course, given my previous post), but the one quibble I have with it (your assertion) is that it (avahi) overlaps functionality with (rhetorical) your router.
I.e., on my network, I can reference either:
1) machinename - which gets resolved by my router (to 192.168.whatever.whatever)
or
2) machinename.local - which (also) gets resolved (by avahi) (to 192.168.whatever.whatever)
Now, not all routers do this, but IME most well-configured ones do. So, I'm not saying that avahi isn't ever needed, but (again, IME), most of the time, it duplicates functionality of the router.
Anyway, regardless of all this, the point is that there's no reason for a newb to disable it.