First of all, I have to acknowledge that mining artifice is some of the most powerful due to the effect on purity. Thing is, getting high purity iron is a lot of work, so for most people it'll be easier to just buy good knives once competition drives their prices down. So for them there isn't all that much benefit in getting a good set of mining clothes, because the mining speed increase isn't that big of a deal. So to stimulate clothes trade I think mining artifice should help other related activities, such as:
- bonus to boulder chipping speed
- bonus to quarrying speed
- increasing chances of getting granite when quarrying
- increasing chances of getting bigger lime boulders
And yeah, M&M already works for the latter two, but I don't see what's wrong with having both artifice and proficiencies help.