"so many pre-recession jobs were simply unnecessary."

Well, they can be looked at as unneccessary now, but at the time they weren't considered to be (otherwise they wouldn't have existed then, as they currently don't now).

It's not as if employers just suddenly "wised up" 3 years ago and said "hey, I don't need any of you - you've been 100% unproductive the past several years, so I will simply lay you off now". Employers are acting as much in their self-interest now as they did 3, 5 and 10 years ago.

In an ideal world what you say is true. Unfortunately, employers don't necessarily know what's the "wise" thing to do. Take the housing boom: from a contractor's point of view it was good business to build many times more houses than the historical aggregate demand. In reality those houses were completely unnecessary: short term gain, long term damage.

Same story for the dot-com (and probably the current) bubble. Employers are notoriously bad at accurately forecasting how many jobs they can actually sustain. And in a downturn it's easy to squeeze more productivity out of fewer employees because it's easier to retain overworked employees.

I think people are picking and choosing what time perspective to use (not necessarily you, but I've heard this sort of thing before).

We can look at job situations 5 years ago and say they were unnecessary, although at the time it wasn't judged to be.

Then we look at now and say the unemployed currently are dead weight. If we weren't good judges of the situation 5 years ago, why are we any better (collectively) now?