At times the ATW trains (especially on the Marches line) can get really busy. The trains are reservable (and seat reservations are put out, unlike on LM) but as has been pointed out, you don't get reservations online for advance tickets (although you do for walk-up tickets).
One of the problems with this service is that they often run 2 coaches. The rest of the time it will be 3. If they put out reservations for all of the advances purchased, it would block out a significant percentage of the total seats on some trains. The answer, of course, is to provide us with a proper service that has enough carriages for the job, but that's not going to happen any time soon.
I've found the staff at Hereford willing to do seat reservations for advance tickets you buy from them, and for any walk-up ticket, but not for advances bought online.