The problem with NC early requires building libraries in every cities you have at that point, and delaying expo is bad for most games.
Okay so this is (much more) a multiplayer perspective, but I agree with you even when it comes to just growing or setting yourself back slightly for an amazing wonder.
I will typically aim to settle three cities (including my capital) before I even think of opening Philosophy, and think that people who really 'rush' National College are neglecting more important things by not understanding how small of a bonus it is.
You basically have two sources of science (unless you have a science pantheon, playing maya, or have a caravan): Palace and Library in your early game. Palace gives you +3 Science, Library gives you 0.5 Science per citizen.
This means if you have a pop 4 city when you start your library you're going to waste quite a few turns to get ... 2 Science per turn. Pop 8 (maybe a nice capital) would be looking at an additional 4 Science per turn. This is irrelevant amounts of science, and it out-strips your potential to produce. You can't really use that extra science unless you really need a far-off wonder because you're still sitting on a fat stack of things you need to finish beforehand. Your capital would gain another 8 Science from building National College (if you already have a Library, which is much cheaper). Again, it just doesn't give you anything hugely substantial because you need the growth to really make use of those new buildings, units, wonders, etc.
Even if you're just looking at a 3 City NC like I often am, you're better off spending that production on Granaries, Caravans, Workers, Defense, etc, etc before looking at Philosophy. Because those bigger cities are just going to catch up on the "early birds" due to their larger size, and quicker production for Libraries and the NC.
Early game food is unbelievably important and you need to grow large cities or expand (in case you're more liberty focused) before investing in science. You're also going to waste a lot less on maintenance if you stack your queue to complete all of your libraries at the same time. Extra brownie points if the turn your Libraries complete you finish Philosophy.