Anyone looked at the vampire spawn stats?
A 6th level minions. 10 hp so they'll drop like flies. But they have +11 to attack and get a +3 bonus to damage for each additional vampire spawn in contact with the target.
So if you have three of these minion on you, you're liable to receive up to 27 hp of damage in one round (45 if you are bloodied).
16 of these are very much a challenge to a level 6 party even though they can be killed in one solid blow.
In 3e low HP always meant low BAB and so these kind of fights couldn't occur (or were waste of time) at high level. Not anymore.
Basically, minions are just a good mechanic way to allow for mass battle that are both manageable (minions go down fast) and challenging. I don't think there's much more than that to read into it.
Incidently, these spawns are perfect for a 'From Dusk 'Til Dawn' scenario...
PS :
Max damage with...
1 Vampire Spawn : 3 (5 Bloodied)
2 Vampire Spawn : 12 (20 Bloodied)
3 Vampire Spawn : 27 (45 Bloodied)
4 Vampire Spawn : 48 (80 Bloodied)
5 Vampire Spawn : 65 (125 Bloodied)
6 Vampire Spawn : 108 (180 Bloodied)
7 Vampire Spawn : 147 (245 Bloodied)
8 Vampire Spawn : 192 (320 Bloodied)
Of course, they are unlikely to all hit even with +11 to attack. Still, do NOT charge into a mass of Vampire Spawn! They are worse than pirahnas!