There are games without npc’s where you can rp (and which I play). However I believe this topic is about game mechanics (which many rp’ers enjoy as well as to rp) and providing feedback to the developers regarding its impact on the player base so I’ll stay on topic.
Many players, from new to old, depend on the availability of the npcs to either quest or train. The older players will learn the timed wandering or short disappearances quite quickly. However for newer players especially the frustration may make or break their interest in the game. If they have spent some time travelling to see the npc this can be doubly frustrating.
As mentioned before there are a number of Hydlaa npcs who have (still) not re-appeared due to the npc server not being fully operational which highlights the problem (I now have 4 quests on hold awaiting their return). Consider, for example, if this list had included Harnquist, Trasok, Levrus and Jayose. I wonder how many players would have logged off or newer players abandoned the game completely? If it had included the new leather trainers (at least one was often disappearing previously for a break) the new leather skills would not have been testable for the last few days.
While npcs taking time away to sleep for example may seem to be more natural I think this needs to be balanced against the added frustrations players may feel, and prolonged absences in particular need to be avoided.