Mentoring still reduces your +hit to 1 making you miss mobs quite often.

Example: Level 57 clicks random instant adventure, gets placed in Seratos group mentored down to level 54. I was doing 2x less damage than I was at level 54 partly because a large portion of my hits were missing while being mentored. This was frustrating and turned me off from running random instant adventures or mentoring down to play with friends that are lower level.