The modifier does work, I use it alot too. BUT its not for SHIPS!! Taking a quick look at the Schema Ships have their own modifier ShipMaintenance!
The code should look like:-
<GlobalCombatMods>
<EffectType>ShipMaintenance</EffectType>
<Scope>Global</Scope>
<Target>
<TargetType>Ship</TargetType>
<TargetQualifier>IsStationed</TargetQualifier>
</Target>
<BonusType>Multiplier</BonusType>
<Value>-0.50</Value>
</GlobalCombatMods>
I am about to test this in game, but if you want to have a play the code is in the GalCiv3GlobalDefs.xml.
EDIT:
Did not work 
EDIT 2:
For my test When starting a game you have 3 ships costing 4 Maintenance, docking ships at planet or SY does not reduce this to 2.
Neither did:-
<GlobalFactionMods>
<EffectType>ShipMaintenance</EffectType>
<Scope>Global</Scope>
<Target>
<TargetType>Ship</TargetType>
<TargetQualifier>IsStationed</TargetQualifier>
</Target>
<BonusType>Multiplier</BonusType>
<Value>-0.50</Value>
</GlobalFactionMods>