If the main is cut away, the RSL pulls the reserve ripcord, deploying the reserve. Since a significant number of fatalities involve jumpers who cut away but do not pull their reserve or pull to late, an RSL is a good idea in many ways. In fact, there are only two situations where an RSL can be a problem. One is in a violent malfunction, where an immediate reserve deployment is equivalent to an unstable deployment. However, a great many skydivers are not aware enough of their altitude to justify taking a delay between cutaway and reserve deployment.
Malfunctions can be extremely disorienting, and can also have a high descent rate.
The average jumper is probably better off getting the emergency procedures done immediately, and an RSL can shave a second or two off the time between cutaway and reserve pull. Another situation where an RSL can be a hazard is in a canopy wrap. In the case of CRW, a person should disconnect the RSL before the jump. Most people who have been in wraps would agree that reaching the RSL disconnect would be difficult in this type of situation. Statistically you are better off with an RSL in this case too.