Even many (but not all) 2nd class tickets bought on the day of travel are cheaper than the daily cost of a rail pass. So, you're paying a whole lot just for the convenience of not having to use the ticket machines, which are about as easy to negotiate as an ATM. This is especially true for regional trains, which do not offer advanced purchase discounts anyway.
To get back to one of the original questions... when I lived in Germany, I always bought a reservation when riding an ICE (Intercity Express, a fast long distance train between large cities). Not to guarantee myself a seat. As the other posters mentioned, seats are almost always available. But, so that I didn't have to waste time walking through the carriages looking for that empty seat. With a reservation, I know exactly which carriage to board and can procede directly to my vacant seat.
Domestic rail service in the Netherlands is all open seating, with no advanced-purchase discounts. An exception is the Thalys highspeed service, which connects Amsterdam to Rotterdam, Antwerpen, Brussels and Paris. This works more like an airline ticket. And with a rail pass, you would need to purchase a reservation and a supplement. I've seen more than a few rail pass holders have to cough up a hefty sum of money to the ticket collector when they learned to their horror that using the pass was not as simple as they had initially presumed.