It's a mixed bag.
Sometimes people get upset that some new forum member posts a thread with a question that's been answered before, and tell them they need to search. But at the same time, if they search and find the answer wasn't sufficient for their problem but otherwise a thread matches their issue entirely, people get pissed they necro'd that thread.
I don't mind it if the necro fits perfectly into the original thread's purpose.
And I can definitely understand when someone necro's a thread that never saw a conclusion and that person wants the exact answer to a problem that fits the mold perfectly. How many times have we seen a thread about a certain issue, people discuss possible solutions, they don't work... and then that poster never returns to the discussion? Hell I'll admit I've been one of those, especially on Windows forums or a thread I can remember on Battlelog for either BF3 or BF4. Issues that may have taken a few weeks or, hell, months to finally narrow down the culprit and fix the issue. (I hate reformatting, and I take it as giving up - I like the challenge of figuring out some strange issue that ends up being a crazy driver conflict... with a damn keyboard driver...)
As I said, a mixed bag, because sometimes a necro is exactly what is called for. But half the time the OP is no longer a participant, and sometimes even PMs don't accomplish anything because the person disabled email notifications and never check that board.