I agree. If already dropped, Do Not Pay. It will return to your credit as a paid charge off (or whatever the negative status was before, only paid) for another 7 years.
Also, once it is dropped from your credit report, you may have trouble paying it as the original creditor has already written the debt off (and has already deducted the loss against their income for IRS purposes) so the likelihood of them having your paperwork still is slim.
If the paper has been sold to a debt collector, they are just trying to collect whatever they can and will continue to try and collect something many years later. I personally do not feel that paying a third party off, especially at full face value, is good for your conscience or anything else, as they just are looking to benefit off of the hard times of someone else.
If the debt nags on you, Pay it forward in some way. At this point, it will present a stronger positive karmic ripple into the universe.
I have worked in the lending and credit industry for 20+ years, and this is my 2 cents.