Azuriel has an interesting post on Parity as Entitlement. He's discussing entitlement, F2P, money and time. At one point his discussion touches on the different amounts of time people have available:
Perhaps this disagreement comes from differing definitions of parity. Tobold in later comments suggests no MMORPG features parity because different people have different amounts of time to spend playing the game. This is not a dilemma to me – as I mentioned previously, the both of us have the same 24 hours in a day in which to allocate our time. I have zero issue with you receiving greater rewards (etc) for having spent more time playing the game than I. In fact, it sort of boggles my mind that this is even a point of contention. Is that not how any activity should inherently work? “You spent more time reading a book and got farther into than I did… unfair!”Many players and MMO developers do not agree with this perspective. If it was feasible to enforce parity of time, many games would do so.
Existing MMOs have many mechanics which push towards parity of time. The most blunt example is raid lockouts. Play a little or play a lot, you can only do the latest raid once per week. WoW even tried limiting attempts per boss. It didn't go so well, but they did try.
Often there are extra rewards for the first instance you run per day, or the first X instances per week. This pushes towards parity of time by front-loading most of the reward onto the first few hours. You still get more reward as you play more hours, but the majority of the reward is concentrated in the first few hours.
The Old Republic does something similar with daily quests. The daily quests can be done each day, but there's also a weekly quest that requires you to do each daily once. The presence of the weekly makes the first set of dailies more rewarding.
Pretty much every currency after the base currency has a cap. Maybe you can only earn 1000 Endgame Currency a week, and can only bank 3000. Again this plays into parity of time. After a threshold, playing more hours simply does not help you.
Finally, there's rest XP. If someone plays fewer hours, the hours they do play become more valuable than the hours played by high-playtime player. The value per hour played effectively scales with the number of hours that are not played.
Far from players and developers accepting the disparity in time played, they actively add mechanics to mitigate that disparity. It is unfeasible to enforce true parity of time, but that doesn't mean that devs and players see the disparity as desirable.
(Admittedly, it would be pretty funny to see a game try to enforce true parity of time. Imagine a game which limited you to 10 hours per week. It would be interesting to see the audience's reactions.)