I don't have any experience with the slic3r build for a Raspberry Pi but I'm pretty sure that it is totally independent of the Cura plugin (i.e. you don't need the Cura plugin).
I believe one should treat the Raspberry Pi / OctoPi / OctoPrint / 3D Printer combo as a dedicated, real-time system. Successful 3D prints require real-time delivery of GCode to the printer. The more "other stuff" you add to the Raspberry Pi, the more likely you are to interfere with the delivery of GCode to your printer.
Most of the slicers available today have nice graphical user interfaces. You can make use of these graphical interfaces to assure that the sliced output will result in a successful print. The OctoPrint Cura plugin has no graphical feedback capabilities you are relying on the .stl file to be properly oriented, sized, etc. so you will have a successful print.
Arguments I've heard for needing a slicer on the Raspberry Pi revolve around having a group (like a classroom) of users with limited compute devices (i.e. tablets). In these cases, using a web-based slicer like IceSL-online is a viable option. See https://www.youtube.com/watch?v=RKZUVxKCdaU for more options.