I can hazard a guess, as much as this will make me sound like I know everything (and I don't). The answer to your question is that people who comment on interlocking do so because it's all they know. Since it's inception, Forge Hub has provided guides to all of the nifty tricks and ways around the standard forging system, and very little was written in the way of level design. Sure, things have changed and more information has been brought to our attention concerning the latter, but the former always remains prominent due to the sheer ease of which to accomplish it. You can immediately see the effects of interlocking, not so much with actual design. Not until you step a virtual foot on the map, that is, in which few of the said commenters do. It's easier to say that the interlocking looks perfect than it would be to give a detailed analysis of why your overall footprint of a map will or won't work. Sadly, it will always be that way. All we can do is keep promoting design over aesthetics and hope that the wall we bash our heads into is made of a soft material.
i believe that interlocking horizontally or on the x and z axis are needed but as for the y or vertical not as much as it requires... floating or resting on other objects. But a sign of a well forged map is interlocking and a person who interlocks, well you can tell that they really care about their map and what it looks like. But there is always special occasions when either one will apply. for example aesthetics