The spell Rusted Blade reads:
You cause a rustlike patina to temporarily cover the target weapon. (Despite its name, the spell can affect any kind of weapon regardless of material.) For the duration of the spell, any living creature damaged by the weapon must succeed on a Fortitude save or contract filth fever (DMG 292). Use the spell's normal save DC for the initial saving throw, but use filth fever's normal DC 12 for later saves. Multiple applications of the disease do not stack, but even a creature that succeeds on one or more saves must continue to save each time it takes damage from the weapon. (Complete Mage, p. 116)
Filth Fever normally has an incubation period of 1d3 days before causing ability damage (1d3 Dex plus 1d3 Con).
Does the spell have this incubation period as well? In this case it would be absolutely useless for player characters: I mean, in case the PCs win, there is probably no opponent left to make a save. In case the opponents win, it is not really important if any of the victorious villains suffers from Filth Fever.
In that case the only use of this spell would be for NPCs who infect PCs.