Players are incentivized to exploit every XP source to gain the most XP. Instead of performing actions because they are inherently enjoyable, they perform them to be as high level as they possibly can be. If the developers want to be "egalitarian", every source of XP has to be monitored, analyzed, charted, and continually balanced throughout the dev cycle. The only cases where I have seen this be successful are cases where "action" XP rewards are so infintessimal compared to quest XP that they are on the scale of a nickel vs. a $100 bill. I.e. the rewards are effectively "feel good" nods to sacred RPG conventions that are statistically irrelevant to advancement.
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