Your seat reservation doesn't entitle you to travel on that train in this class without a valid ticket. You will get a fine if you try to travel with just a seat reservation. And if you don't pay or are catched multiple times they will initiate criminal proceedings against you
but just entering the ticket number when doing a reservation would be so incredibly easy to add. as it is now, i can buy up seat reservations without even being on the train, forcing multiple people to stand.
It's more complicated because there's also time limited (or even unlimited) tickets/passes, but I agree it could be done.
Seat reservations expire 15 minutes after passing the station. By now all trains have those small displays on the seats which show reservations. They simply turn off after 15 minutes, indicating the seat can be taken by anyone.
There are less seats in the carriage, it is more comfy, you get at-seat dining service, free newspapers, faster Internet, more generally people who behave better and are more quiet, and if you have a flexible ticket Lounge Access