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.