Thoughts on the PA Oil, Gas and Mineral Rights Addendum

I am looking for some thoughts on the PA Oil, Gas and Mineral Rights Addendum from Pennsylvania agents who are working in active Marcellus Shale areas.

The standard PA Agreement of Sale provides that all OGM rights to a property will be transferred to the buyer as part of the sale. If certain rights have been transferred to someone else by the current landowner or a prior owner, those rights cannot be transferred to the current buyer. That seems like it would be enough, but I have been seeing contracts with OGM Addendums. This addendum includes a checkbox for the buyer to specifically waive their right to investigate exactly what those transferred rights are, or not.

My questions are... Wouldn't the standard agreement of sale cover the need for urban and suburban residential properties? or are you using the OGM Addendum on all contracts? Have you ever had anyone that did want to investigate the OGM rights? Who did you send them to and what was the cost involved?