Not true, at least outside the US where using other sizes is easy. It all "just works", so it's used all the time.
Age 5, my school work book is B4 size. I can glue the a A4 sheet (or two A5 sheets) the teacher hands out.
A little later, and the work books are B5 sized. A4 sheets fit in if folded.
I want to make a poster at home. I set the document size to A2, design it, press print, and four A4 sheets are printed, exactly the right size. I can change it to any other A-size very easily.
I want to photocopy / print some A4 documents, but save paper. Two (or four) sheets fix neatly on one page. Or, I can enlarge it to A3 — most photocopiers in most offices have A3 paper in one of the drawers for this. There's an A3 laser printer in my office, it's useful.
From my desk, I can see things laser-printed by me in A4 and A3; things produced in A5 and A6; and notebooks in B5. If I need a poster in A1 or A2 size there's a printer upstairs.
- a few flyers (A5) - much more convenient for people to hand me in the street, big enough to get information across on, not to big to big cumbersome for me and (presumably) cheaper than A4 to print in bulk
- a postcard (A6) from a colleague who went on a lengthy vacation
- business cards - some (but admittedly not all) of which are A8
- and lots of A4 - it's what I print out when I need a hardcopy.
I don't currently have any A3 on my desk, but I've used it in the past when I have complicated diagrams to analyze and want to get it all on one sheet.
I often print out documents as 2 (or sometimes even 4) pages to one A4 sheet to make it easier (and more tree-friendly) to read away from my computer - this works OK because the aspect ratio remains the same.
