Here is an idea based off TheVedis's idea...

Rank & Class specific Repair Kits

Examples...

Repair Kit - 10 (Blacksmith)
Repair Kit - 40 (Leatherworker)
Repair Kit - 30 (Armorer)

Now every craft would have these repair kits (except cooking, unless they start to do repairs) The numbers next to the repair kit is the maximum rank it will repair. So a repair kit 40 can repair 1 to 40... This helps with how many different ranked repair kits you need to carry around if you chose to do so. These would also work based on Optimal Rank (not repairable rank)

The requirement would be (if you were using the repair kit) Would be to have the repair material on you. This gives non-crafter's the ability to repair anywhere. Now these cant be 100% repair, just like normal crafters, as even if we are 40 ranks above an item, we still fail sometimes... But maybe we make them 75% success rates.

This insures that rare repair materials are still needed, while giving people the ability to repair at any location at their expense whenever. This also gives crafter's something to craft and sell, still making us useful. This will probably end up costing the end user more gil in the long run, but it brings convenience into the equation.