It's done for both reasons.
Each builder has their own preferences, but personally I use the segmented mobs mostly for mobs that tend to appear in groups, like 'a pack of wolves' or 'a flock of sharks' or 'a group of teenagers', where each individual segment would be called 'a wolf', 'a shark' or ' the boy/girl'. Makes for a nice and compact display in the room, but shouldn't really be too much of a surprise, since the desc of the main mob gives it out. The more 'traditional' segmented mobs that I've made, like Scylla and the Hydra, are scripted however, since they have features that the segmented mobs don't provide. Like the Hydra sprouting two more heads for each one you cut off, or the heads of Scylla actually invading passing ships (which is what they did in the old stories).
And when it comes to guards I prefer the attacked mob screaming for help and loading the helpers. But, like I said, all Builders have different preferences.
One day I might even get around to making the tentacles of Sarlacc in the Great Pit of Sarkoon a segmented mob - which I think they should be.
