I think nicknaming is creative, and in general looks nicer. Not doing it would be like getting a Dog and literally referring to it as CANINE. I sort of get this guy
>>31000961, but it's also pretty autistic and inconsistent. I don't know why people put legendaries on pedestals all the time, they probably would "enjoy" the nickname - and as an MC wouldn't it show that the association you have with the legendary isn't just purely because it's on a need-to-use basis, but because you're actually close to it like a regular pokemon. Something that other trainers would be super jealous of, because not only can they not encounter them, they will never have a bond where they can nickname something that strong.
I know that is pretty autistic, I don't think you need such a deep reason to nickname at all. I'm just explaining how it's JUST AS autistic to have standard boring rules because you're pedantic about keeping things in-line.
Gotta have a middleground.
I nickname legendaries cause it looks cute or makes sense, I don't have any other reason why really. If I don't nickname something it's probably because I don't really care about the species much or can't find a good name yet.
If you happen to like the default names, then sure keep them. But if you're just doing it to satisfy some autistic need to keep everything in box-format then... That's pretty fucking autistic.