The reasoning is that disturbing them sets them back, and I have read that it then takes two or three days before they are settled in and back on track. following that reasoning they will be bringing in less honey while they re organize and get back to work...
I have a few problems with what is said above.
While I agree that a full blown inspection and rearranging frames/boxes etc WOULD set them back a day or two.... Doing a quick check for cells and looking at the bottom of the frames by tilting the box, seeing if they need more supers etc and putting them back together is not going to set them back more than an afternoon. They don't need to reorganize, because you didn't disturb the brood nest or storage, you just LOOKED to make sure they are not preparing charged queen cells.
It takes 9 days for them to close up a queen cell. In general a hive will swarm about the time, or shortly after those cells are capped. So on a two week inspection schedule the bees can cap and swarm before your aware they are making plans.
The small amount of disruption you create by looking is not going to set back the honey production a tenth as much as it will if they swarm.