I assume the downside of the autopilot is that it only travels in a straight line at the maximum sustainable straight-line speed, so a manual approach could save a couple of minutes per trip, plus avoid interdictions. Plus it takes a module slot (and power?), of course, which could certainly be used better for something else.
Those seem pretty significant disadvantages to me.
power isnt really a thing tho is it when in supercruise? and power management would just shut it off when weapons are deployed. in principle i am not against it (though would be far better if it were crew doing it imo) but imo FD need to be making the game more hands on, and harder for botters not easier for them.
once we can get out of our seats and have other stuff to do whilst the ship is flying then there will be a much stronger reason for it.... but to me, in a game which is currently fundamentally about flying our ship, then makign the ship fly itself is removing the experience not adding to it.
NOTE this is not a "i want to make the game hard to get into".... I applaud better ingame instructions, making the hud better etc etc etc...