I think armadas and troops not costing any maintenance hurts the long-term strategic planning of the game. Right now a military buildup is totally normal, and it means that a race can be wiped out before we can even see the war start.
Now, I think we all agree that war declarations should automatically pause the game and maybe even prompt you to cancel your current dispatch mission. However, wading into a war that's already started should be a last resort. We're supposed to be masterminds here: we should see the war coming and try to either stop it or redirect it.
If a race had to pay maintenance on its armadas, and even higher maintenance when they were stationed in orbit around a hostile world, there would be a real strategic tradeoff to building up the fleets. Build too many armadas and you're not able to spend on stuff that will make you stronger in the long-term, like your science and RCI buildings. Build too few and pin all of your hopes on the late game though, and you'll find your planet hosting a new Thoraxian hive.
If armadas cost maintenance, a large military buildup would send ripples through the solar system. Neighbors would see the growth and respond in kind. Now in possession of new, expensive hardware, races will start looking to make those fleets pay for themselves, so everyone definitely doesn't want to be seen as the easy target. Mutual defense pacts and non-aggression pacts can help calm the tension, but if races aren't already friendly they might need a credible force willing to work the backchannels for them...