I'm a novice at RTWs (thinking about one; never been on one; fairly new FT member; etc) but, from what I understand, you don't have to use it just for a single trip.

For example, you said you wanted to go to Paris and Lima and I assume from jpatokal's post that you're based in SEA. You could do on a RTW:

SEA-Lima-Paris-SEA (with additional stops as required)

and book a Lima-SEA-Lima ticket so that you break up the RTW. You therefore get out of it separate trips to Lima (twice) and Paris.

As jpatokal says, you have to compare the cost of that ticket and the RTW ticket to the cost of just buying a return ticket each time.