Hi Pavel,
This is your opinion and I respect it. But if it's intended for trying css, why do you have the addon options like font size, font weight etc available? They also go in the inline css.
So if those options can go inline, I think it's alright for the rest of the code to go there too. The point is, as a developer, I want all the css to be in one place.
It's confusing to ask customers in the text addon for example, to change the font size, line-height in the addon options and then go to the custom.css file to change letter spacing or text shadow, don't you agree?
Also you have the option to enable production mode in Sp Page Builder, which can export all your css in an external file, so it is not pointless at all, it actually helps you to not create thousands of useless custom classes and you have all your css in each place. When a customer is purchasing your template, he does not need it all, he removes what is not needed, thus the css of the addons too. When you have everything in a custom.css file, all the css of the removed addons remains there, it will take a lot of work to find and remove it.
But this is not even the point. My point is the frontend editor should reflect exactly the frontend of the website, especially if you don't have a backend editor. I cannot make changes in a frontend editor and then have to refresh the website to check if they applied correctly.