I still have no clue what the pioneer exploit is.
The pioneer exploit refers to the fact that when you order the production of a pioneer, thirty population is subtracted from your city's current population. If you subsequently cancel that pioneer, that thirty population is returned to the city. Since city growth is partially dependent on its population, you can add pioneers to the production queue to keep the city population below the food limit (and potentially keep the population low enough to allow for the maximum possible growth in a given city) and then prevent the pioneers from completing their training, thereby storing population beyond what the city is really capable of supporting, which can potentially mean that you can rapidly boost the level of your cities each time that they gain a sufficient amount of food to reach the next level simply by cancelling the training of a bunch of pioneers. Then you would add in a new batch of pioneers, bringing your city population back down so it grows faster, allowing you to do this again the next time you have enough food to increase the level of the city.
As for the subject of the population cost making outposts not worthwhile, I would tend to say that population has little enough value to me as the player that expending thirty of it on an outpost isn't that much of a problem. In most of my games, I'll have at least one or two cities that don't have the food to advance to the next level but do have the population to allow me to churn out a few pioneers, so those cities will build a pioneer as I need them, unless there's something I consider more useful that they could be building instead. Cities which have the food to advance to the next population level are allowed to grow naturally to that population level unless I need the pioneers quickly or unless training the pioneers wouldn't significantly increase the time it takes the city to advance a population level (note that growth due to food is essentially max(1, min(3, food_surplus_%/100)), so if you're relying on food for growth a Pioneer unit sets you back at least 10 turns for the next population level, unless you're taking advantage of the exploit described above). Since I usually have a city or two which cannot reach its next population level, I usually don't order pioneers in cities which can reach their next population level because I usually don't need many pioneers all at once and would usually rather have a city advance to the next population level than have a spare pioneer, nor do I make use of the exploit described above to hasten my city growth as I don't think it's worthwhile (it's a single-player game, and the AI isn't so strong as to make me need to abuse exploits to defeat it, so I don't see any reason to bother with ordering and cancelling pioneers to gain a slight growth advantage). If I don't have a city which cannot achieve its next population level, then it's a matter of which city type I need to advance more as to where I get my pioneers from, and that depends entirely on the game type I'm playing.
As for whether or not Outposts are sufficiently worthwhile compared to cities to justify expending a pioneer to build an outpost - that depends on the quality of the settlement locations available to me. A 1/3/0 site with no nearby resources isn't going to tempt me very much if I can grab a couple world resources somewhere else with an outpost unless the 1/3/0 site is in a strategically valuable location. On the other hand, it would take a lot of world resources to make me pass up a 3/3/3 settlement site temporarily just so I could put down an outpost and grab the resources (note - this depends greatly on my own needs for resources at the time - if it's the only crystal crag I've found on the map, I might just take it instead of the 3/3/3 settlement with the pioneer I have in the area even if there aren't any other resources nearby, and send a new pioneer out to grab the 3/3/3 location later, but that also depends on how far away the crystal crag is from the settlement site, the monsters in the area, and whether or not I think the AI might be coming to plant a city or outpost in the area, as well as whether or not I'm intending to make use of crystal items on my units any time soon). I do think that outposts are a bit too weak at holding the land they claim, as cities can push their borders away and they don't have even a single unit of militia serving as a garrison even with all the upgrades, meaning that anything can take your outposts away even if the unit is of as little combat value as a pioneer is, unless you happen to keep an army of some kind in the area, and while the bonuses outpost upgrades offer are useful, I don't generally find it worthwhile to tie up a city with the construction unless I have nothing better for that city to do, especially as it's very easy to take an outpost and I'd rather not allow my enemies to gain the benefits of a mostly-upgraded outpost when I'm not in position to stop them from taking it.
In short, I view population as a more or less worthless resource except in that it gets me occasional special city structures which are very variable in value, and in that it limits the rate at which I can expand my empire, though rarely as much as the surrounding monsters do. Thus, I don't particularly mind spending 30 population on an outpost, though due to outposts' lack of security and lesser ability to hold ground, I'd sooner make a city. Since population has nothing to do with income, prestige, research, production, or your maximum army size, and only affects your expansion (slightly, because usually I'm more limited by monster presence than by pioneer production) and access to a few special city structures, it isn't a resource I'm overly concerned about expending.
