Well the thing about positive air pressure (+AP) setups is that they're not +AP, they're just bad airflow set ups. That +AP they're trying to create pulling in more air than is being pushed out? Runs into a wee problem in that cases aren't airtight so all that 'extra' air just flows out all the gaps and seams and that's dumb as dogshit as you want to all that nice 'cool' air to flow across the cooling devices mounted on the hot spots on your hardware.
Neither positive pressure nor negative pressure really matters, because as you already pointed out, PC cases aren't airtight. You'll never really get a high amount of either positive or negative pressure because PCs have so many vents and cracks for the air to get in or escape. Air follows the path of least resistance though, so for the most part it will try to go where the fans are.
If you have a slightly positive air pressure it helps prevent dust from getting into your PC and keeps things cleaner because air is getting pushed out of the cracks rather than pulled in, and all the places where you're pulling air in are probably covered by a filter. On the other hand if you have slightly negative air pressure then you end up with dust getting sucked in through the cracks and vents around the case rather than getting sucked into where the filters are.
In terms of long term performance positive pressure is better because it keeps your computer more dust free and prevents things like the fine stack on your CPU cooler from getting too caked in dust.
Whether the air actually gets to the components it needs to inside the case doesn't have to do with positive or negative pressure, but with the positioning of the fans. You use the fans to create a specific airflow path inside the case that most of the air follows.