The trigger "tampon" made of fiberous material will eventually degrade from solvent.
And wetting the "tampon" with any type of liquid (i.e. water, oil, solvent) will reduce its vibration dampening ability when fired in that condition, which will negatively affect the life of the trigger spring.
Yes, but ...
Getting exposed to some occasional solvent/CLP/lubricant is a different thing than getting continually drenched in something, and remaining wet, especially if it's a strong solvent. (Why would that be needed for something other than the barrel, though?)
When they were explaining how the fiber insert would still work if exposed to unexpected liquids (like water submersion), it was explained from the perspective of it being an exigent situation, not a condition where the liquid was continually present in excessive amounts for long term. Once the exigency was over, it was expected that the gun would be decontaminated/cleaned, and a new trigger bar spring installed, if needed (damaged or didn't dry out).
It was even mentioned at one point that if the weapon had to be kept in the field for some duration of an emergency, and the fiber insert became so wet it slipped out of the spring, that a 'field expedient' replacement could be used, in the form of a tightly rolled cigarette filter stuffed into the spring.
Well, the inserts in the current springs look tightly enough contained within the springs at each end, that I'd not be overly concerned about losing one (but I don't have any of the older ones left over from the early Sigma days to make a comparison of any spring revisions that may have occurred).
Besides, since their best guesstimate offered back then was that the springs might fatigue without an insert within 3K rounds being fired, and I'd think it would be easy enough to get a new spring in a working gun quickly enough.
Also, the standard replacement recommendation for duty guns is to replace the spring every 5K rounds fired, which is a conservative recommendation for dedicated defensive guns. They acknowledge the trigger bar springs (and other springs) are lasting longer than that in comp guns.
As far as a plastic insert? Good question. One we asked way back then, too. After all, the Colt AR extractor spring uses one or another density of plastic insert, right? The answer was that the fiber one worked fine for the intended application and duration, and the trigger spring design is obviously different than the extractor spring in an AR (or the aftermarket trigger spring widget designed for the Beretta 96 many years ago).
Of course, if many owners and issued users didn't often
drench their pistols with solvents, CLP's & lubes for "cleaning", we'd not have to worry about excessive exposure of liquids to critical areas like striker assemblies & striker channels, extractors, trigger springs w/fiber inserts, bores, magazine bodies, etc.
I swear, some people seem to think they can use the same 'hose down and drench' cleaning methods they use on their cars, when cleaning guns.
Of course, some folks don't seem to be inclined to read the directions and precautions on cleaning & lubrication product labels, either. Now that we have more gun products that are less toxic (not to mention more environmentally friendly and less 'aromatic'), it makes sense to reduce needless expose to products which might risk damage to guns (like plastics, or holding the tubes holding tritium vials in sights, etc), as well as reduce some potential risk health risks to people.
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, but it's not a problem if it gets exposed to solvent, oil or CLP. Most owners/ users forget it's even there once it gets grey or black from powder fouling. 
