Allow me to say, first of all, that this post is not intended to be sarcastic or mocking. I mean what I say. I would not take the position suggested alone for my own campaign, but I offer this as an idea to deal with a social issue that some apparently seem to have.
Something said by Carl Nash on the last post caught my attention:
"I don't care too much one way or the other about rate of PC level advancement, but my players do... and it is not uncommon at all for several sessions to go by without a combat as the PCs are busy doing all kinds of other things in my world. With our schedules leading to infrequent sessions, if I leveled up by the book there would be basically no advancement at all for years of real life at a time."
Personally, I have never experienced this 'busy-ness.' The most time-intensive period of my life was when I was going to university, holding down a job, raising my 5-year-old daughter and taking care of my physically-impaired spouse. This included attending classes, studying, taking my daughter to school, bringing her home, cooking and cleaning the house, putting my wife to bed at night and working day shifts in a restaurant when I was not at school. I had help, of course; two sets of grandparents to help look after my daughter and a part-time nurse to give my wife her meals twice during the day and keeping her clean and healthy.
And I still ran D&D. Hell, I would have gone mad without that release.
But I do understand that people can't or more precisely won't adjust their schedules if it challenges the possibility of promotion, acquisition or perceived responsibility to a child's sports or academic advancement. I grew up in a time when my participation in sports happened in the immediate community, which could be enabled by walking to the rink, pitch or diamond, where we would gather to be driven to visitor games in three or four cars. I know that today every parent takes every child individually in a separate car to a game and that many such activities that used to exist in every neighborhood now demand journeys of up to fifty miles or more by car. Moreover, the helicopter-mentality in the raising of children has meant that parenthood is by far a more time-intensive activity than it was for my parents or for me.
So I won't judge or condemn. You and your players, for whatever reason, are condemned by circumstance.
I propose, however, that rather than attempting to link experience-for-play to some artificial standard of game play, that the experience should be directly linked to the actual time that the player spends attending the campaign.
Theoretically, any DM could establish that the experience gained by the players for their characters should be equal to 10 points per level per minute of playing the game. This would mean that a first level fighter needing 2,000 xp (sorry, I'm perpetually stuck in e1) would need to play for 3 hours and 20 minutes in order to advance to second level; it would then take 1 hour and 40 minutes to advance to 3rd, 2 hours and 13 minutes of play to advance to 4th, 4 hours and 10 minutes to advance to 5th and so on.
Thus, the players, regardless of what activity they participated in, combat, gaining treasure, talking the green swamp monster into helping the town build its walls, whatever, would be rewarded for just showing up. No other specific behaviour would be required. This would enable the participants in the campaign to consciously be aware that their failure to attend would mean definitively falling behind others in the group - and yet they would know that by adjusting their schedules somewhat, two or three runnings would advance their characters at least one level.
Of course, once passing 5th level the time necessary to advance each level according to the arithmetic I've proposed would get quite lengthy - but undoubtedly an exponential increase of some kind, which the individual could work out for their own campaign, would solve that problem.
As I say, not meant sarcastically. If advancement is this important and circumstances for play all that unfriendly, then why not just tie the experience level to the level of participation?