Schematic drawings are useful for many reasons. When we migrated to D-Tools from Quickbooks estimates, once of the biggest selling factors of D-tools was the ability to keep track of all of the parts required to build a successful project. I know there are a lot of us that believe that we can build these systems in our head so its easier to cost estimate a project without all of the design work, but D-tools has helped us keep a better eye on the little details that were commonly missed, (i.e. connectors, adaptors, etc.). IMO The "detailed" schematics should really only be done once you have been awarded the project though. Otherwise you run the risk of giving away a lot of design labor for free. If you are just using the basic line diagrams for estimation, that is a great way to go. The completed schematics help your technicians in the field accomplish the installation faster and troubleshooting goes faster if they have a complete picture of the system without having to trace wire. I think we all know that you can give the same 10 AV components to two different technicians and get completely different results. It also can help prevent return trips because of a missed adapter or wrong cable. You take a lot of the guess work out for them. Also, with all of the EDID and digital signal management that needs to happen these days, some projects we have worked on would not have been possible without the schematics.
That's my two cents.... well maybe three cents.