As a Christmas present, I got a sampling of old-school modules written and published by various sources around the OSR. To one degree or another, all of them are excellent and one of my favorites (The People of the Pit) has already been integrated into my current campaign. When I was reading them, however, I noticed something that bothered me. The modules written for those rulesets that included the thief class — 1ed, OSRIC and Labyrinth Lord, for example — had some encounters that virtually required a thief to get past them unscathed. It made me remember why I dislike the class, but allowed me to view it from a new perspective.
When the thief class is introduced, player freedom is limited. The mechanical assumption is that the thief is the only class that can accomplish the tasks described under thief skills. Thus, traps suddenly require a thief to bypass. Players who choose to play a different class are no longer able to find or disarm traps.
Let me illustrate why I hate this using a different class — the cleric. In an encounter with the undead, clerics have at their disposal the ability to Turn. This ability, however, does not prevent any of the other classes from doing damage to the undead or otherwise participating in the encounter. If we were to apply the mechanical assumptions of the thief class to the cleric class, only clerics would be able to engage the undead and do any harm to them. All other classes would be completely ineffective against them. Encounters with the undead would suddenly be rendered completely unfun, which is how I feel about traps when thieves are allowed in the game.
Thus, thief skills, skill systems, or any mechanic that prevents anyone without access to a particular skill or mechanic from solving or otherwise defeating an encounter is not fun to play with. From a design perspective, every encounter in the game ought to be doable, solvable or able to be defeated by everybody — but some classes will be better equipped to deal with certain situations than others. Therein is the fun of the game — choosing a path (a class and its particular ability-set) and trying to overcome all of the various challenges adventures throw at us.
Where does this leave me in regards to the Thief? I still don't like the class, in large part due to where it takes the game mechanically. I am no longer going to say 'no,' however. As long as I don't get lazy designing traps — making sure I know how they work — then I can use the Thief skills as one might use a saving throw. I'll make the player describe to me how they might go about finding and removing a trap (or whatever other skills they might use) just as I would any other player in the same situation. When they fail to describe to me a legitimate way to succeed, the player of the Thief would then (and only then) get to roll against his skill. This way everybody can survive an encounter with a trap, Thieves are just better equipped to do so.
9 hours ago