Save your res
Case 1: Ammo / Med spam
If you spam 5 meds to save 1 marine, and he kills the skulk, you've wasted meds. What's even worse is if the marine dies. Perfect your aim so that you can med almost on the marine to save res.
Same for ammo. If a marine wants ammo, don't spam 5 packs for him. Most likely he'll die before he uses it.
Dropping 2 packs will suffice, unless he's doing something like gunning OCs or RTs down, in which case you may drop more.
Case 2: Weapon Spam
Imagine you have 7 marines.
You drop 7 shotguns/HMGs for them.
They move out to the hive.
A skulk kills 1 half way to the hive.
As everyone has a weapon, the weapon is wasted and not picked up.
If you dropped 3-4 weapons for 7 marines instead, the weapons will continually be recycled until there are more guns than marines.
SGs/HMGs/GLs are good, but remember that the LMG excels in the medium range damage category. A full shottie/hmg team won't kill a fade blinking down the corridor, but a LMG has the cone to do so.