Being a porting skeptic, I've asked some of the sport's large canines why their guns are ported. The universal reply was that while they doubted porting's ability to yield anything positive, they didn't want to take a chance on not having every small advantage their competitors might have. One told me the same thing about barrel work. For the average shooter, none of those things really matter much as we do not place our shot with the same precision as those guys.
Most porting haters got that way from shooting beside ported guns that spewed combustion ash on them and/or forced them to push their ear protection in extra-tight. It does increase the amount of combustion dirt in the air as well as the report of the gun while delivering little in the way of benefits. Shoot a ported shotgun with a breeze blowing in your face and you'll feel the fallout.
The more smoke a powder produces while burning, the more porting looks like it is really doing something and cold weather will exaggerate that appearance. Truth be told, ballistic experts will tell you that shotgun target loads do not generate enough combustion pressure for porting to be effective. I once owned two Krieghoff KS-5s that were identical except one was ported. No one who shot both guns back to back could tell any difference.
Does it reduce a gun's value? Yes for some people; no for others. Either way, if you bought the gun for a fair price, it's a no harm, no foul deal.
Ed