With 1u the clearence for the rails is taken away from the long side of the module, e.g., you have less space to work with compared to a 3u, given the same size front plate.
This also results in more space for your fingers! :)
You cannot just give a 3u module a quarter turn and declare it a 1u, you will need a new layout to fit it in the narrow space.
Actually, if the module has been properly designed, you can. Be prepared to have quite a wide 1U module though.
Both 3U and 1U versions of my LB5 module use the same PCB, and only have different front panels, so the "lost" area needed for the rails virtually has no impact.
About whether a 1U only case makes sense / is a good idea or not (or if the 1U format makes sense at all), will only depend on how one would use it, so there's no universal answer, and it's not for everybody.
Even though the short demo used this case on its own, the idea was more to complement existing setups to add a couple 1U modules. Some are only available in 1U format, some interfaces (like the steppy) are more user friendly / natural with modules in the horizontal orientations, so there can be uses for such cases. And nobody will force anyone to buy one. :)
There were also some similar reactions when I released the 1U/3U adapters, and yet, I always need to make more. I understand that this solution too is not for everyone though.
Another thing to take into account is the type of modules to be used in 1U : the utilities I released earlier this year were originally designed as 1U modules only, and I finally decided to adapt these to 3U (using a specific PCB for each format this time). So far sales are slightly higher for the 1U format, and this was kind of expected, based on what happened with LB5 already. I do apply the same prices for both formats, and that's just the way it should be.
One could also argue about if hardware modular synthesizers in general make any sense in the 21st century when you can use VCVrack instead! ;)
I guess sane people just don't invest in modular synths. :)