Biased opinion from me.
I prefer (most of the time) overseed here in southern Cal.
But, like anything else, it depends on if it is done well.
Of course, the same can be said of not overseeding. I believe it was Tom D who mentioned drainage
issues for fairways seriously having an effect on dormant fairways. In socal, so many courses drain poorly, and have
poor soils, that recovery from the rains we get in winter is slow, and sloppy. Add poor weed control, and you get
uneven, spotty turf with a Heinz 57 of grasses that leads to a pretty poor surface.
We have 36 holes, overseed one course pretty much wall to wall (not greens, they are Poa). On the other course, no overseed except roughs (?). The dormant fairways are well taken care of, and poa and weed control is really done well. Even with that, after our big rains, the course is just now coming around to playing nicely. The overseeded course recovered in a couple of days, and had many more rounds than the dormant. In fairness, there usually a little more play on the overseeded course, but it was significant after the rain