We fly into Munich and will travel to St. Goar..... river trip to Bacahrach...travel to Trier from St. Goar...Two adults and 17 year old son. Should we buy the pass? Youth and Twin?
"Should we buy the pass?" That totally depends on how you want to travel. Do you want to go from Munich straight to St. Goar, as fast as possible? Then maybe. But there is a lot to see between Munich and St. Goar, and if you take your time, the trips might be too short for a railpass to be economical. Where else are you going? You don't need a railpass for St. Goar to Trier.
If you do decide on railpasses, sounds like a German Rail Twin, 2nd class, for the adults and a Youth for your son would be about right.
As long as you leave after 9 AM, all three of you can use a Rheinland-Pfalz-Ticket for €26, total, to go from St. Goar to Trier. That's less than €9 pP; not worth using a railpass day for that.
Also, if your only other use of a railpass is to go from Munich to St. Goar, full fare is only about $150. That wouldn't justify a railpass, either. What's more, if you book early, online, from Bahn.de, you can get a Dauer-Spezial fare for as little as €29 pP, and ride the fastest express trains. I checked for one day in April, and the Dauer fares were sold out, but they did have fares for €49 pP.