http://www.science.uwaterloo.ca/~cchieh/cact/c120/limitn.html says
Chemical reaction equations give the ideal stoichiometric relationship among reactants and products.
However, the reactants for a reaction in an experiment are not necessarily a stoichiometric mixture. In a chemical reaction, reactants that are not use up when the reaction is finished are called excess reagents. The reagent that is completely used up or reacted is called the limiting reagent, because its quantity limit the amount of products formed.
So the term "excess" seems to be a term of the chemistry art meaning that part that is not consumed in the initial reaction, rather than meaning excessive. I think every reagent in a reaction is classed as either limiting, stoichiometric, or excess. Excess in this case does not mean unwanted or non-useful, if I read correctly.
One other consideration comes to mind. When you want the chlorine to react to something, you don't want it to be too stable. For continual injection into the water, I think there is probably a 99:1 or more dilution. In my particular case, the solution is only diluted about 1:1. So I don't want my solution to be too stable. My solution is only injected for 4 minutes during regeneration, and it is not put into the treated water.