~ or do you think they should have been NWPs, with thieves automatically gaining and improving them (as the ranger gains & improves tracking)?
That's essentially what we did sometime in 1e when the NWP proficiencies came along in the Oriental Adventures, Dungeoneers & Wilderness survival guides.
If you weren't a Thief (or an Assassin) you could spend 1 NWP slot & pick up any one of the Thief skills at LV1 % chance of success (dex/race/armor mods applied as normal). You could increase your base % by spending additional NWP slots. So for ex, if you wanted to pick locks as well as a 3rd lv thief? Then you'd have to spend 3 NWP slots.
For the Thieves and assassins? They got all of their listed skills automatically (so no change) & if they did nothing would automatically increase their % as listed whenever they gained levels (so no change).
Like any other character they could choose to spend NWP slots on these skills. So a 1st lv Thief who invested 2 NWP in say Pick Pockets? Would increase their % to that of a 3rd lv. When they gained 2nd lv? Then their PP would increase to 4th lv %s & all their other skills would increase as usual.
We did something similar with the Rangers tracking.
Rangers remained unchanged - unless they invested NWP.
Other characters simply gained the rangers base % per NWP invested.
This almost never came up.
So when 2e arrived? We just kept doing it this way. I forget how many %pts we assigned to each NwP & what mods we applied to tracking.