Buy it if you have the coins. No reason to not have one of each exotic when the opportunity presents itself. I find it most useful for PVP but not much of a Crucible player to be honest. In PVE the main time you would use it is during the Crota fight. Me personally, I'd rather save the exotic slot for something else. There are plenty of ways to regen health and a Red Death is not going to help take Crota down any faster. If you don't have armor with the health infusion perk, it's helpful to switch to RD and kill a thrall for health regen.