I knew that the Rules Cyclopedia had a skill system for PCs, but even though I owned an RC back in the day, it never was used for play. Most of what I researched online proved that using the skill system seems to be a mixed bag with players across various forums and such, as is the Weapon Mastery system. But going back to the Skill system, as I read through the skills in the Darokin guide, I came to the realization that trainable skills, other than class abilities, for PCs in early editions of D&D through AD&D 2nd edition, really weren't a good fit.
An example of this would be a smithing skill or glass blowing, which are two of the ones I read over. These skills are highly specific, you can use these skills to essentially make or repair weapons/armor, or spend time making glassware; in my mind though, these are the skills for NPCs. Making or repairing weapons or taking the time to actually blow molten glass into various wares, would be one that is time consuming. I would assume it may take days or even weeks to make really good, quality items. But if you're plumbing the depths of dungeons or wandering across the vast wilderness in search of treasure, you really don't have time to stand for days or weeks in front of a forge or kiln. You're ADVENTURERS...go adventure.
The game is called Dungeons and Dragons, not Goods and Services.
If a party can be self-sufficient in certain tasks, then in my mind, that takes away the prospect of NPC interaction, which then robs the players of the opportunity to both interact with and, to an extent, help develop the characters who reside within that adventure or campaign world. No, I think it suits the player much better if the characters are essentially ignorant of the world in which they live. Player characters should know only very little, until both the characters and players learn more through playing of the game.
Now, even with that, I DO think that having a single background, or as AD&D 1e/2e had them listed, a secondary skill is fine. This mechanic gives the PC some sort of prior history before becoming an adventurer, even if it's just a minor thing. I like how DCC handles this with zero-level. Players roll on a chart randomly to see what sort of zero-level character they start with, then, assuming they survive the funnel, move on to become a 1st level classed character. With something like a simple background or secondary skill, a player could potentially gain a slight advantage in one area of expertise or knowledge to which it applies, but as they are no longer pursuing that life path, it doesn't become a focus for the character. A character who was formally a weaponsmith may be able to haggle a bit and get a better deal when buying a new blade from a local NPC, or maybe the trapper has some basic knowledge of animal lore that can help find enough food for the party, if rations are running low.
To me, this just keeps everything simple and easy, making for a more enjoyable and free flowing game.
