It's not unrealistic for an unexperienced farmer to make multiple suits of workable plate mail. Ned Kelly, for example.
--> That armor is designed to stop black powder firearms and would probably be very difficult to actually fight in.
I seriously doubt that, in the process of making four suits, he somehow destroyed fifty kilograms of metal.
--> This I agree with, although you have picked a bad example. I don't think anyone has said metal ought to be lost, only that a deformed/shoddy product made and become available for melting.
What people seem to be focusing on is material loss, which doesn't seem to be the OP's actual focus. Most of the "pro-fail" types seem to agree that material destruction would need to be handled on an individual basis. Also, I forget who it was but I agree that outright failure should be confined to Dabbler (50%?) and Novice (15%) [numbers my own] and should take vastly longer to perform the task than a professional. Perhaps 10x as long. This produces an opportunity cost for sudden profession shifts.
HOWEVER ...it is my feeling that if that was implemented, it would be
absolutely necessary to have some form of apprenticeship system in place to allow players to minimize or eliminate material waste. Also! Persons without skill in appropriate categories but with that labor enabled should automatically seek out an appropriate craftsdwarf to apprentice themselves to. They could perform hauling duties to increase productivity while also gaining some experience in their new field.
For reasons which should be obvious, we regard information as easy to acquire. In the world of DF there is no internet, no phones, and barely books. (Gutenburg 1448, 15th century) Urist McPeasant can't simply whip out his Blackberry and google "gem cutting" and become an instant expert like we can. I suspect it would be difficult for us even WITH such assistance in many cases.
Not only does he have to build the tools (from memory, assuming he has seen a gem cutter at some point in his life) and remember how to use them (slightly easier at least on an extremely fundamental level) but also invent several lifetimes of tips, tricks, rules of thumb and points of tradecraft to become proficient or highly skilled.
The "trouble" in this case is not an annoyance; it is a realistic simulation that provides the player with several things:
1. The starting 7's skillsets will be more meaningful.
2. Immigrants with new skills will be valuable additions.
3. Training a dwarf in a skill becomes an investment and something to plan around.
4. Additional social and economic options (Master-Apprentice relationship)
5. Probable increase in productivity because Urist McLegendary no longer has to go get his own stone from the bowels of the fortress, his apprentice will clean his workshop of clutter and fetch stone for him.
...or at least that is what I support and envision.