For newsletter style stuff, nah. The "confirm your registration" email can "pay" to get past the wall, and then you're done - approved pair established, future letters can probably be zero cost and everyone receives the same one.

I wholly admit that this is arguing theoretical setups and that's always problematic, but of course patterns would be established pretty quickly. There are loads of simple tactics that would still make spam dramatically harder, and legitimate use nearly unaffected. The current reputation system has clear, massive gaps that really don't need to exist.