I guess it's just a smart thing to do that I never really thought about. Killing those Needlers won't take most 70s too long and Zangarmarsh is along the way to Blade's Edge, anyway. Making a short stopover to take a chance on the Firefly is a prudent move as far as making money is concerned. It doesn't take too much time and the gray item drops are worth a decent amount when sold to vendors. Making money is easy if you make short stops along the way, such as fishing the pools of Pure Water on the way to wherever you're going for guaranteed Motes of Water. Engineers can do the same thing by extracting Motes of Air from the gas clouds in Nagrand while doing The Multiphase Survey in Osh'gun. Sure, they'll probably need to keep swapping goggles, but it doesn't hurt and gives players more loose change.
Sometimes my wife takes quite a long time to meet up with me at a rendezvous point because she stops to mine or herb or fish (depending on the toon she's using). She never lets a yellow dot on her mini-map go while I'm just too darn lazy to mine anything that isn't Khorium or Adamantite. Of course, now that I've dropped Mining, I have an excuse not to make any stopovers. But stopping to gather or kill specific mobs is excellent advice for players who want to earn Gold beyond doing the numerous dailies. It doesn't take much time and it's not out of the way. It just takes a little diligence and knowing a good route to take.
When you're on your way somewhere, it pays -- sometimes literally -- to make short stopovers. I don't even mean farming or gathering, specifically, but making sure to do things on the way to a quest or instance. These days I encourage my party to do our Sunfury Attack Plans in Manaforge Coruu, killing Sunfury Researchers for the quest drop. While Formula: Enchant Weapon - Major Intellect hasn't dropped for me just yet, it's only a matter of time. Although the drop I'm after is a Bind-on-Pickup, there's certainly Gold to be mined (pardon the pun) when stopping to do things that are along the way.