I've played a lot of zone, and I imagine that the idea is that the 3-2 at a -2 balances out the largest disadvantage of that zone type, i.e. having only two rebounders near the basket. I think people overthink the -2 thing a bit, particularly if your guards and SF are faster than the opposition. There is no reason they can't close out to shooters. Speed is underrated in the zone defense I think.
The 2-3 zone has natural "holes" on the perimeter, so unless you extend the zone, it is just more difficult to defend the 3. I've always found extending the 2-3 zone to some +X works well, provided I have some good defenders at SF, PF, and especially C if the zone gets penetrated.
Thus, I think the reason you might have heard that the 3-2 (-2) and 2-3 (+2) zones are conventional wisdom is that those are natural attempts to mitigate the large disadvantage of each zone type. For the 3-2, it is rebounding, so sag. For the 2-3, it is perimeter shooting, so extend. All of this is subject to change, of course, depending on your team and your opponent. I would say they are not bad starting points.