Swarms don't make attack rolls.
Most swarms appear to have no CMB either.
So what is the point of Weapon Finesse on, say, a centipede swarm?
Assuming this is a mistake, what would be a nasty replacement feat?
It could simply be an option by which the creature gets the feat it is entitled to, but does not actually gain any power. It could be a purposeful design choice due to the sometimes onerous requirements in combat against swarms.
My guess is it may have been a situation where someone quickly assigned feats to creatures based on stats, maybe even done using a script. When the script found a creature with a very high Dex in comparison to a low Str, it automatically assigned Weapon Finesse.
Which works beautifully for everything BUT swarms.
I just threw Dodge on the thing as a replacement.
Vermin Typed swarms are mindless, and thus have no feats typically.
If a swarm is made up of small enough base creatures, maybe they included the bonus Weapon Finesse Feat that the CRB says all creatures below a certain size gain for free?
Hmm, that could be it actually. The Weapon Finesse does have a mark beside it in the entry, it just doesn't say what that mark is for.
If its a little B, then it tends to mean Bonus Feat.
Hope it helped!
Hmmm, so being a bonus feat, it can't be swapped out then, correct?
Ah, good to know. That will prevent me from making encounters unnecessarily difficult.
So Weapon Finesse stays on the swarm, in all it's worthless glory.
Elven Curve Blade FTW
In my next session: a spider swarm wielding a rapier.
I think, for the preservation of realism, that all of the tiny spiders should be wielding tiny rapiers.
True, anything else would break verisimilitude!
Now how to give them 8WF?