In Morrowind and Oblivion using the pre-made classes wasn't recommended by experienced players, as they often overloaded skills using certain attributes, and left others off entirely, or didn't have a good mix between the major and minor skills.
Once I learned the system I always used the custom class creator to pick my specialties and major/minor skills because you could level more effectively. In MW this wasn't such a huge issue, because scaling was not a thing in that game, but it was an issue in OB, where enemies had scaling, and if you consistently had inefficient leveling it was possible to end up with a character that was very under-powered relative to the scaled enemies.
I understand that they wanted to create a more intuitive system in Skyrim, where there was no such thing as "misc" skills which would not increase your level if you raised them, and while this was not a bad idea, it lead primarily to people basically raising every single skill and doing literally every quest line on the one character because the game allowed you to do so, all you had to do was grind a little and you could master magic, or stealth, even if you started out as a bruiser. But, it also led to poor situations where you could grind to level 30 on smithing, enchanting, and alchemy, and then get destroyed the next time you went dungeon crawling because you spent all your time leveling in crafts instead of fighting.
I'm not a big fan of the jack-of-all-trades Dragonborn, and i played that game several times with differently skilled characters, and an alternate start mod. Also, I personally played around with a skill change mod, where you picked major/minor skills that got a small boost at the beginning and leveled faster, while your misc skills leveled much slower, effectively making it like the older games, but not actually restricting your misc skills from contributing to your leveling. I liked it, it was a very good mod.
So I can appreciate the love for a "classless" system, or at least a system that allows you to effectively make your own class by picking from a set of skills and specialties, but asking for such a thing in a D&D game, a system traditionally built using fixed classes, is a foolish undertaking.
