It's like .... all dwarves and duergars know where is the big mad dwarf with the full set of good dwarven eq. All gnomes know where to go for the full set of the things which are only usable by gnomes.

Though it would be much better if there was also another centaur npc for the people of midlle levels.

However if people want to find some special things like the mark of centaur crusaders, then they would have to travel somewhere deep into very dangerous place what is very far away from the common lands.

Edited: And only because centaurs are the seasonal race