It is a valid point based off if what we see is
only from the Terran perspective. The Terrans do serve a purpose to the storyline, it's just that they're unaware of it. It is only made clear later (or earlier, if you read the manual backstory) that the Zerg want (not
need as people incorrectly assume) the Terran psionic potential as an ace in the hole against the Protoss. Essentially the Terrans are depicted as being in the 'wrong place at the wrong time' from their perspective (which is useful as this perspective lends itself well to introducing the world) but later it is revealed that the Terrans are actually in the 'right place at the right time' from the Zerg perspective.