Forts do increase gold income of the province by a percentage equal to the administration bonus. Not that significant for your average province yes, but the right farm or gold mine can get a nice boost.

And I find it hard to believe you were that gold starved when Marignon has several 10 gold 20+resource heavy infantry options to make good use of extra forts. Knights of the chalice are cool and stuff, but line infantry is important too to, well, hold the line. I guess that was a critical fault in Marignon/Pythium plan, spamming knights is a late game strategy because indeed an early economy cannot sustain full production of them in two forts. Less knights but more forts and more heavy infantry and things may've turned quite different.

Not to mention, with just two forts Marignon ended up quite vulnerable to being completely locked out of income (and production).

Setting up forts early and often is critical. I believe one of they key factors for our victory was precisely managing to stay ahead in number of forts all game long, meaning even when some got sieged I was still able to pump out reinforcements. Caelum was second in forts pretty much all the time, just behind us. Those were not coincidences.
Hmm.. i do suspect that bonus is so small it wont matter most of the time. At least not unless your on a gold mine.
But yeah, inheritet a moderately sized force of knights and crossbowmen. The Knights seemed pretty badass, and it was rather easy to dump all my gold into recruting a bit of those as well as crossbow men and mages.

You are wrong about the last bit though. Marignon did not have much spare place to put down another fort that would also need defending.
And by the time those were locked down the final, decisive battle had already been lost. A gamble had been taken in a losing war against waves after wave of berserker giants.
The last fort had actually not been taken though. It was torn down to deny as many resources as possible to the enemy