On the note of the Collectors they were being rather dumb, but it did actually kind of fit them. They were controlled by Harbinger who is a Reaper, and it's noted that the biggest weakness of the Reapers from the start is their arrogance. They legitimately believe that nothing can stop them, which is why Sovereign risked uniting the entire galaxy by doing a full out assault on the Citadel. It never occurred to it that it might lose, especially with the Geth on its side. It was a Reaper, and Reapers were supposed to be unstoppable.
and so Harbinger commands the Collectors to start constructing a Human Reaper because again, it never occurs to Harbinger that they might lose. Despite Sovereign being destroyed, Harbinger still believed that the Reapers as a whole could not be stopped even if they lost a few along the way.
The whole battle strategy of the Reapers in all 3 games is dumb because they didn't believe it was possible for them to fail due to their overwhelming numbers and technological advantage. They also very nearly win despite the fact that they had a horrible strategy.
At no point in the series were the Reapers ever really shown to have good tactical thinking, and they were the ones controlling the Collectors so why would they have good tactical thinking?
I don't know if I should call that well done or not, because I've honestly no idea if all that was intentional or just another case of "Probably best you don't over think it" that happened to fit when you actually did. Over thinking it just happens to be fun sometimes =P
That said I still think, awesome as they were, the Collectors served no real purpose for the overarching plot of the trilogy. They tell us nothing new about the Reapers, and only gave us a lore point about what happened to the Protheans which is interesting but wasn't really relevant to the main plot. Come Mass Effect 3 they're not heard from again outside of MP due to high fan demand for them.