I have done my share of internals. Some of the time you find valuable clues in administrative audit logs and bash_history. Clues that mean you make a good and impactful finding within the timeframe of your timebox. Reality is an attacker with this level of access is going to have more time than I will, but it still means a measurable impact to security because it decreased the time it took to find something. So in that sense the OP is correct, it comes down to a risk reward trade off and is it worth clearing the history to potentially buy yourself more time in the event you are owned. It isn't as black and white as "well, the server is owned, of course you have my bash_history, that is the least of my worries now..." it is, but it could also be the one thing that gives you 3 extra hours to detect and shut down an attack.