Well, I guess we disagree, and in all likelihood once I've run more than three sessions perhaps I'll realize my folly. But - and here I'm not focusing on the thief/halfling thing - it has already caused confusion. My players ask "So if I don't spend any of my luck how high can it get?" - as the DM I can give the standard answer I should give in cases like this "Here is my ruling now, but I reserve the right to change it", but I do think that it doesn't reflect well on the book. The question came from a NON-thief, because they need to be more careful with its use.
I just don't understand this, really. IMNSHO, the answer is "18, like any other stat." Done. Why is this such a big deal?
It's just a case where it seems like GG erred on the side of too little information - they have provided a mechanic for regaining/adding Luck, but have not explained how that should work (particularly for non-thieves). The game has some spells for which there are complete pages of detailed results on exactly what happens. I would have given up a spell or two for more detail on one of the core abilities.
Hmm, I disagree. The mechanic is explicitly detailed... Luck exists to ensure characters are played consistently with their alignment. Non-thieves spend Luck on a 1:1 basis permanently, and only GAIN Luck when it's granted by the Judge for... ARRPEEING their alignment. For example, there's a +1 bonus to Luck for certain actions in Sailors on the Starless Sea. There's also a -1 penalty to Luck for a certain curse in People of the Pit.
Now, returning to the thief/halfling - that aggravates the lack of info because this incompletely explained attribute is one of the core / most valuable traits of two classes.
So, if I understand the complaint/question, it's something like this: What happens when a Thief/Halfling earns a +1 Luck bonus as a result of good ARRPEE? I think Raven_Crowking's answer is to just give them a +1 like any other class. Typically, because a Thief or Halfling will have spent Luck in the course of the adventure, this is unlikely to result in them earning a permanent +1 bonus to Luck (i.e. the max increases by 1). I think that's the way it's supposed to work, and that will serve to keep a Thief from maxing Luck out at 18 relatively quickly. But, again, as you say... the game is deadly, and a Thief is very likely to have been spending Luck left and right. So it's already balanced. A Thief who can refrain from spending Luck and still survive long enough to max out at 18 deserves everything he's got (again, it's just +3! bonus for him, but could be made more advantageous if you allow Lucky Rolls to float with modifier [I do]). Again, IMNSHO, YMMV, etc.