Ultimately it comes down to:
if it is appropriate for many people who do not trade (participate in the marketplace) to have an influence on the trust system
Marketplace participation would be even easier to game than your theoretical merit circle. Go to the marketplace, post a thread, have your alts post in it, post some trade feedback... eligible to have influence now?
No to mention that business is done and scams happen outside of marketplace too (gambling, ICOs, etc). Merits at least can't be produced out of thin air by any random newbie and all transactions are public.