If there were a system like the one you suggested with unique buffs, gear, mounts, etc it would in practice be more about getting those items than it would be about helping someone. People would make alt characters and Guardian themselves and they would spam around town and harass newbies to try and get them to be their disciple.
The best mentors are those that aren't looking for any sort of reward and the system we have now I think is the better solution.
(Edited for autocorrect mistakes!)