I used to use them on all my guns. Generally, the do diminish the sound. But the problem with paintball is that the caliber is large (relative to firearms), and the ranges in paintball are fairly small.
And typically the style you see in paintball is the expansion-type (ie green scrubbies, etc). They make no attempt to cancel the sound wave using elliptical baffles. So, the intead result is you can still hear them....
A silenced poppet gun is similar to an unsilenced spool valve, etc. Legally they are allowed, but its a grey area. You can see in the link above that someone was given a -20- year prison sentance for building an airgun silencer. It was won on appeal, but its still pretty scary. I also own lots of firearms, and I do not want to -ever- give the ATF a reason to deny me of my rights.
__________________ www.montneel.com "the evidence strongly suggests that neither Billy nor Adam (Smart Parts) could have invented the electronic paintgun" -Garr M. King, U.S. Judge