My opinion about the "non-adventuring skills" (Craft, Profession and Perform) is not that they are necessary or desired in the game because you want to encourage the PC's to sit around weaving baskets for profit. I think they are fun and useful because they give you other angles for insights into the adventure as it unfolds. A PC with Blacksmithing as a skill might notice things about the weapons or armor the bad guys were using that is a clue to their origins and helps add detail to the adventure and world.
The question of course arises, "Yes but why do I need a skill for that? I can just tell the PC the information I want them to know if they said they were a blacksmith as part of their background." That is true. But there are some reasons why I might want to attach a number to that blacksmith background.
One is to simply make such skills work consistantly with the rest of the system. A PC might have the History skill but that doesn't mean that I automatically tell them the answer whenever they have a question about history. I have them roll. I see no reason to treat blacksmithing as an auto-success every time.
Another reason to codify this is about how to deal with a PC that has a very broad background as described by the player. If they say, "Well I was kind of a jack of all trades around town. I did some carpentry, some farming, worked as a cook, did a stint down in the mines and sometimes I played my lute for loose change at the inn in the evenings." So do you let that PC auto-succeed on anything having to do with carpentry, farming, cooking, mining and playing a lute? Seems like he's got a pretty big advantage over the player whose PC just knows how to blacksmith.
Finally there is the issue that, if you don't assign a number to it that every blacksmith in the world is equally skilled. That's not a problem if they simply need to hammer out the dents in some armor or make some iron flatware. But what if they need to craft "armor fit for a king"? Or a "mithril blade suitable for enchanting"? Again, a GM is free to adjudicate that on the fly based simply on the PC's background and other factors. But, to me, having a number assigned to it is handy.
Fortunately for me this was the easiest house rule that I ever came up with. It goes like this:
At character creation, answer the following two questions about your character. "What did you do to earn a living before you became an adventurer?" and "What do you enjoy doing when you aren't adventuring?" The answers to those questions become skills in which you are trained. An appicable ability score will be agreed upon by player and GM.
Every 4 levels (at 4th, 8th, 12th...) you get a Background Skill Pick. This may be used to either select a new Trained background skill or to add Skill Focus in a background skill in which you are already trained.
For me it was as easy as that. And I'm really glad I did this too because the background skills have added some real interest to our game. The Dragonborn Ranger used to work in a quarry and was able to discern some interesting things about when and how the mines they passed through recently were dug, just as one example.
