With my purchaser hat on, most of the hired machine shop "salesmen" I've dealt with were worse than useless. They generally didn't know much and just got in the way, playing telephone between me and the person who actually had the answer to my question. The good sales guys for machine shops were the ones who had been at the company in another capacity and then went into sales. They knew what their company could and couldn't do.
With my business owner hat on, it seems like the big disadvantage of commissioned sales reps paid a % of revenue would be that their incentive is to bring in business, regardless of how profitable it is. How do companies deal with this? If you're selling widgets, there's presumably a price list, but with machining, there's a lot of guesstimating as to what my cost will be.