tying up the fleets reduces the Blitzkrieg//Sneak Attack thing that Stardock doesn't like that much--only a player who has a vast advantage in numbers could pull it off so effectively if the fleets have to stay near the planets for a few turns to ensure victory.
Sorry, what do you mean by that?
Fleet A (Player 1) is taking over Planet Q (Player 2).
Takeover lasts... 3 weeks, let's say as an example.
Week 1: Fleet A's Transports enter orbit, deploy troops. Player 2 has nothing aside from the planet available, at the moment. Let's say Player 1 has 2000 troops, Planet Q has 6000. (Player 1's advantage is 5:1, so he should win by almost double).
Week 2: Player 1 moves Fleet A with Backup Transports B to assault Planet R. Player 2 seizes the opportunity and moves in with (Ships (A), Transports (

).
A: Player 2 continues to destroy the undefended transports (considered orbiting Planet Q) from Player 1. Player 1's advantage drops from 5:1 to 3:1, making the battle about even.
B: Player 2 deploys 2000 troops, making the ratio 1:4 (troop numbers). The advantage changes because troop transports add advantage (as I said before). Advantage is now (say) 3.5:1, making Player 2 the leader.
Week 3: Player 1 realizes he made a mistake. (Maybe he sends in more troops, now, maybe he doesn't). ((BALANCING DISCUSSION: If Player 2 changes conditions, realistically and to be fair, Player 1 should be able to react. There must be a limit, however, or else players will wind up sending everything they can as backup (epic, but unproductive). Maybe each situational change adds 1 week to the battle, to a maximum of 6 weeks. Something like that.))
Therefore, with this system, Player 1 now has to keep some sort of defense fleet in the area to try and keep Player 2 from sending reinforcements (or attacking the transports), or risk losing the battle even though he had the advantage (5:1 Advantage factor, 1:3 troop factor).
Stardock has done things in the past (engine nerfage) and proposed others (right-of-passage treaty) to try and prevent the player from committing the sneak attack/Blitzkrieg strategy. If the players fleets are tied up for a (slightly) longer time with each planet, only the player that has a fleet for every two of his opponent's planets can safely attempt to overtake the opponent in two weeks (and it won't be two weeks, it'll be two planet battles).