In the name of science, Rentahamster, DrDogg, and I spent a few lazy hours running around Thanalan killing every dodo we could find.
Our scientific experiment yielded the following data:
(1) There is no penalty to XP or SP for having a high level character stand there and tank all the damage while low level characters plink a mob to death.
(2) XP is distributed uniformly to party members based on their level regardless of whether they actually participated in the fight or not by doing damage, healing party members, or buffing/debuffing. If they were around the fight when it happened, they get XP.
(3) SP is distributed to party members based on a currently unknown algorithm which counts damage dealt per member. The highest damage dealer gets the most SP. (Everyone roll a gladiator at launch!!) The more members which deal damage to the enemy, the lower the total reward gets. Members which deal no damage to the enemy receive no SP reward. Healers can now commence raging at SE. :lol
(4) SP rewards drop significantly from highest damage dealer to lowest. Often times, the guy who dealt 60-75% of the damage got some SP, the guy who dealt 25-40% of the damage got zero.
In conclusion: The system for distributing XP and SP in this game is really fucked up and poorly thought out. If you plan to be a healer in this game, prepare to spend time leveling your mage job on your own because healing won't give you anything to level your job. SE really are trolling what few healers there are already if FFXI is any indication. If you actually play a healer, people will offer you disgusting sexual favors to join their parties because leveling a healer is going to be pure agony. :lol