Retainers - Overlooking a Rule Again

The other evening I rolled up a bunch of potential retainers for my old & improved West Marches campaign on 3x5 cards. And looking once more at the rules in the Mentzer Basic Set, I again noticed a rule I never used back in the day but had noted when I did my Cover to Cover series.

Once hired, a retainer will serve until their contract expires (as set when negotiating their service) OR until they gain a level.

Frank even made a note that a retainer just shy of gaining a level may ask to stay on just long enough to level up (not that anyone should know in character how far they are from gaining a level, or even that there are levels to gain for most classes, or that killing monsters and taking their stuff is the way to gain levels... so there are a few holes in the concept -- but I like it!).

I guess that's what, in AD&D terms, would separate a retainer from a henchman. The henchman is there come rain or shine, while the retainer is just there to make a buck and if possible gain a level of experience.

Recently, when I have had players with hired sellswords or porters and the like, I have the hirelings make a morale check at the end of the adventure to see if they are willing to stay on. I think I need to remember this "quit after gaining a level" rule, as it means the players will need to renegotiate their contracts if the retainer becomes more powerful/capable if the player wants to keep that retainer around.

I'm looking forward to the extra bit of characterization and complication that having retainers will bring to the game.