The futility of holding the middle
I have found that the late game on larger maps leads to the problem of a "no man's land in space"
The middle of the map tends to be 6+ jumps from the homeworld of both players, severely affecting allegiance. This is not a big problem for the advent and the allure of the unity, but when half or more of the planets income (resources, not tax) is being wasted those planets are not very productive and therefore not very valuable.
Usually, after breaking the deadlock, i won't even bother colonizing the planets i bomb. The necessary upgrades and fortifications mean that the extra income of capturing the planet will take longer to pay for itself than the game might last.
At higher levels of fleet supply the majority of income is lost anyway; this, combined with the lessened income of allegiance-defficient planets, makes planets nearly detrimental to capture.
Credit income rarely suffers as trade income is usually much higher than tax income by late game.
Resource extractors are nowhere near as effective at increasing resource income as trade ports are with credits.
So, keeping in mind the expense of building and maintaing a combat-capable fleet, what is the point of the middle other than to say "I have more planets"?