With @aristath work on this ticket #25160 I think we need to consider theme developers that use this type of CSS for block alignment. A big issue we have now is nesting paragraphs/headings. You may have a situation where you have a block that is full width and has a bg color on it and inside that you might want to put a paragraph that is aligned wide. Well with this setup your paragraph will go to the smallest declared wrapper size, I'm constantly adding align-wide and align-full classes to my paragraph blocks manually right now to get past this. It doesn't need to be on by default, just a way to declare that you want wide and full alignment available on all blocks.

GB is assuming that all theme developers are using the same CSS for block alignment but do not give a default standard that everyone should be using.