If you read between the lines, this is the very reason why Id released the source code as GPL instead of BSD: They didn't want their competitors to take their engine, make proprietary improvements, and deprive Id of revenue using their own work.

They never open-sourced any of their engines until after they had published a game with the next iteration of their engine.

Sure, but that just means they had multiple methods of ensuring they had created a suitable moat against competitors, the GPL being just one of them.

They didn't release any of this code as open-source until they'd wrung every dime of engine licensing out of it and it had been completely obsoleted.

It's free-as-in-couch software.

1) If I and said did not collaborate, then it will not help me, or anyone else.

2) If said corp refuse to collaborate with me and will use my code for free instead, then it will not help me, or anyone else, except said corp.

Why I should care about (2)?

Because - as it happens fairly frequently in BSD land - said corp might change their minds and contribute back, code and/or money.

Ok, when they change their minds they can start using GPL code with no issues.

Less time wasted on free support requests would be one way.

1) it helps their competitors

2) they may change their mind

3) they may pay for dual licensing (in case of work by a single author)

In general interests of unfriendly corporations and mine are more likely to be opposite than shared.

The answer is in the question.

Not really.