At Hamilton, paying a line sitter (or covering the line sitter's ticket) reduces your cost over what the secondary market charges, or even an $849 premium may cost, and there are no rush tickets there, so there is no guarantee, so the line sitter is risking a lot more time, as some tickets free up minutes before showtime.
It isn't really comparable to DEH, especially since you are already buying heavily-discounted tickets, but more importantly, you are taking tickets away from the people behind you in line. When I didn't know if anyone was able to join me for a show that night, I would only buy one ticket, mainly because I wanted as many people in line to benefit.
That said, I have also been known to resell tickets on StubHub/Ticketmaster/etc., but usually if I'm not going or if I bought extras to cover my costs. One time, I heard the people next to me complaining about what they paid, and I held up my ticket, and said, "Let me guess... this guy screwed you, too?" while pointing to my name on the ticket, heh.
I just have less concern for people who can afford paying hundreds extra vs. ripping off the people trying to experience things on the cheap in line directly behind me.