PvP and RP are not usually seen as complimentary, but when done right they can really enhance things. The spar system is an immersion and flow breaker, but what other option is there? Simply steal the PK or Pk lite system from Asheron's Call.
Doing a optional quest or simply entering a command allowed people to flag themselves to attack/be attacked by anyone else flagged the same way. Want to turn it back off, just do another quest or enter a different command/die.
I think a simple system like this could really benefit players with a variety of RP styles. The people playing bandit types, people in story lines with a lot of interpersonal conflict, and people who would just want to use an occasional staff strike for emphasis.
Remember, Boromir did not say to Frodo "Give me the ring! Do you want to spar? 10 9 8 7 6 ..."