Neutering planets actually increases the total number of ships it receives.
A little more complicated than that. Wormhole Guard Posts count, and if I remember correctly, the 'best' point is at 11 reinforcement locations in a system (Actually, it's AIP/10, but caps at 11). Since Wormhole Guard Posts are placed at wormholes, the base locations count after neutering is the number of wormholes. Defensive AI types get +1 Wormhole Guardpost per wormhole, and higher difficulties get +1 (or more?) Wormhold Guard Posts per wormholes.
So a system with 6 wormholes, owned by a Turtle AI set at diff 9, will have 3 Wormhole GPs per wormhole, for a total of 18 Guardposts even after the system is neutered.
On top of that, killing non-Wormhole Guard Posts has the effect of lowering the planet's population cap. Planets at the pop cap get no/fewer reinforcements. Cap is (Guardposts + 1) * 100(200 min, 700 max).
See this thread (and post, specifically) for more details:
http://www.arcengames.com/forums/index.php/topic,10047.msg97601.html#msg97601