One of the huge problems with this is that you cannot attack your own planets to increase the IP there.
There are many situations, then, when you are the owner with only a sliver of IP but cannot increase your hold there because you do not own it. Yes it is possible now to own a planet with 5% IP while the enemy has 49% IP there, and be unable to attack it. The moment the enemy takes it they'll have something near 70% control uncontested.
The other problem with removing nuetral is the spec-to-win thing (or even just no non-allied players being online). Right now you can overcome spec-to-win by threatening to attack the neutral worlds, which means players will have to play or risk losing them.