Multiple RCS increases how fast a vessel turns and responds to your commands. However they are very CPU intensive so are usually limited when people design for CPU requirements. However reasons you may see more then a couple in a build include. 1) Made before CPU or not intended to work with CPU. 2)Related to 1 is that RCS used to determine all of your maneuverability but this changed (i think that changed in A11) to be more about Thruster Type and Placement. 3)The Ship needs the extra maneuverability for its intended function, especially true if the build is for use with CPU enabled. Although somebody playing based purely on game provided information is likely to include at least one (i think the Tooltip still says required on all vessels for turning). A lot of modern (A12) builds include few or no RCS. However certain things such as wanting to dodge instead of tank hits, or have faster tracking for using fixed weapons are considered important enough for people to include multiple RCS, sometimes this is the case despite the CPU Cost.