Having all of the relevant drawings, documents, etc. is always invaluable when you are conducting your survey and deciding on which monuments to hang your hat on. This has always been an issue in the surveying industy and will probably not get any better. However, remember that the field data and the product that derived from it is the property of the client. And without his/her permission, you cannot simply hand that information over to any other party.
Consider this...Neighbor A is suing Neighbor B for whatever reason. You are Neighbor C. Before the case makes it to court, they settle. You see an opportunity to cash in as well. You know the attorney for Neighbor B personally. You call the attorney for Neighbor B and request a copy of what the issue was about and what the settlement was.
Do you think the attorney simply gave the information up without obtaining permission from his client. If he did, surely he would be brought in front of the bar. As professionals, I believe it our responsibility to treat that type of information as an attorney or doctor would his client or patient's information.
Of course, if the map is of public record, then all bets are off.