One of the biggest lessons I've learned as a MMO tourist is to never finish your to do list. In general, the fun goals - e.g. run every dungeon at least once - get done early, while the less fun goals tend to get put off til later. When I stick around "too long", I find that I walk away from the game with less positive feelings and stay gone for longer as a result.
I'm at one of these crossroads in EQ2 at the moment - a bit over halfway through the non-raid dungeon progression, nearing the AA cap, and approaching the maximum on most of the current expansion factions (most of which I don't really need for anything, other than a passtime while I look for groups and some free AAXP). Groups are getting a bit harder to find as I move away from the easy dungeons of the expansion, and the side projects I can do while I try to get a group are getting less interesting (e.g. grinding Desert of Flames factions for an additional housing option, now that we can have multi-housing).
My subscription happens to be due, and now is about the time to cancel if I want to take advantage of the "winback" promotions that will most likely follow November's expansion - this year, the same amount of money spent on EQ2 would have netted me $20 worth of station cash and a vulture mount if I had planned my gaming schedule around EQ2's marketing promotions. In and of itself, that's not necessarily reason to re-arrange my schedule. Then again, if it happens to encourage me to take a break at a time that makes sense anyway, a little EQ2 intermission might not be a bad thing for everyone concerned.
Specifics of this case aside, I wonder if there is a niche for a game that actually plans on players wandering off every few months. A Tale in the Desert is notable for doing something almost like the season finale of a TV show, actually bringing down the servers at the end of a "telling". Obviously, no developer wants to not get paid during the "season break", and perhaps the effect on the community would be disruptive. Then again, there could be long-term gains in avoiding player burnout, and I could see the new trend of story-driven MMO's - SWTOR chief amongst them - being well positioned to taken advantage of a tourist-driven playerbase that will naturally wax and wane as content is released.