Personally I think this practice of pilots paying for hours on airliners engaged in commercial operations should be stopped by legal means. It is blatantly NOT "line training", they are sitting in the right hand seat as an acting first officer and should be getting paid at least minimum wage. Also it means that an airline involved in this type of exploitation will not have a decent level of experience in the RHS due to constant turnover of F/O's.
Another consideration is that you have a potential situation where you have a first officer who is not up to standard but sits there because of their wallet and it creates an unfair market balance as the airline has reduced their crew wage bill considerably compared to their competitors.
Also worth noting that one of the airlines in Europe who operate this "scheme" have had a fatal accident with a large loss in life in the last 12 months.
Rant over, I think I've said this numerous times on here and the wannabes thread before.