You get more sp/exp for fighting harder enemies, you get less sp/exp for fighting weaker enemies. What you are suggesting is giving out a flat SP/EXP reward for completing a repeatable quest. That's a bad idea. It goes back to doing easier guildleves and it also further reduces incentive to kill anything else outside of guildleves. This means less reward for greater risk.
How's that for education? Or did you miss the school bus again?