Only that this example is outdated. Quarries are enhanced now by Castles and Cathedrals.
EDIT. But basically the question is this. How many tiles do you think your city is going to work late game? Because in the previous three releases, a Tradition capital ended up working just 3-4 tiles. Normally those with huge food (wheat in a farmed cluster, bananas in jungles, towns). So, those 3-4 GPTI that I placed near capital to enhaced it, were mostly wasted.
Secondary cities make better use of those tiles, because it's not so easy for them to work on specialists, but the rule still applies: If a specialist give more raw yields than the field tiles, city switch to work on food tiles and specialists, ignoring your other yields.
Now, consider that Tradition finisher adds +1 food to all GPTI. This makes a GPTI worthier to be worked, and if you place it over a resource, the benefits of making sure that this tile is going to be worked are better than the yields lost for the other improvement.
EDIT 2. Oh, and food has been drastically reduced in the last two releases, so it could be that now it's not that easy to work on specialists.