Its just the fact that the markets will be inflated to a point where the economy crashes it will be the same thing that happened in REAL LIFE to the global economy, same principles. Since there is no risk vs reward (you get reward 100% sure you lose the materials for the successful synth but you get an item to sell for compensation).
I hope I am being clear on this, as this is the only thing that's really a big loop hole and I can guarantee Yoshi-P will not do this if there is no chance of losing the materials. He is all about risk vs reward, and frankly not to be a sour-pus, I think they [dev team] know that it would affect the economy for the worse.
It's a great idea, really it is. But even without giving exp, even with limiting it with Leve Quest count (which is kind of laughable since they changed it, you get leves so quickly now) the fact that you can dupe the system by crafting tough items with no risk (even if its not HQ) you can/will be able to flood the markets, driving the price down, making it less demanding, stalemating the economy, crashing the stability. Just like that, because you know everyone is going to be doing it, not just one or two people.
And finally, the problem with making it not 100% successful is, that people might not want to take the chances, granted they could possibly make it where the poster of the ad can require the crafter to have XX skill in their craft? That might fix it, but there would always be chance of failure and some people wouldn't want to risk the risk.
Edit: But you make the assumption that you only craft for people you don't know, you have to think like a criminal in order to protect yourself from the criminal--so to speak. If I was trying to exploit the system, I would charge very little if anything, and have a friend or myself craft the item, then sell it in the Markets because I'm guaranteed the item 100%. Once everyone begins this you have the under cutters to make the profit, until it becomes cheaper to just buy the item on in the MW because its been cut so much that it might cost slightly more or is equal to buying the mats (and you'd still have to paid the crafter if you did the suggested idea).
Then following that path people would just stop using this system save for all the crafters who are crafting items they are just going to sell on the Market Wards, creating self-obsolescence.