The pros of adding sound dampeners are reduced road noise, reduced panel rattles, reduction of sound transferred outside the vehicle, etc. The cons are cost, weight, and the labor intensive nature of its installation.
Some people claim they've had luck with it. Others have applied it to their roof and come out one morning to tar all over their interior when it released itself. I personally wouldn't try peel n seal but it's a choice one makes.
Peel n Seal is asphalt. Butyl rubber based mats are technically superior in many ways. But some people still feel the price difference warrants the choice. I installed/used asphalt for years, but Ive gone to butyl and wont ever look back.
2x Peal and seal is a waste of time and effort. Also, if you do choose to use a Butyl based deadener, always remember to clean your surfaces with acetone as to remove any oils left behind from the manufactuer of the vehcile.