I think out of the box most people like a medium pen. You get a smoother experience due to greater ink flow. You get a bolder line than you would with a typical ball point pen. I think for the average, non enthusiast this has been determined to be the equivalent of one size fits all.
I used to like F when I first started with fountain pens. Now I like 1.1MM or above stubs. They are smooth, put down a lot of colorful ink and make my cursive look better than it should. I mostly write notes and drafts so I have no issue with the size of my writing. If I was filling out a form and vintage manifold F would be my choice.
I think your last sentence encapsulates much of my feeling.
I admit that a wider nib gives a more beautiful line. If all I did with my pens was to write personal notes on unruled stationery, I would probably use very wide nibs for the visual effect, and just write much larger.
However, I use my fountain pens as my main daily workhorses. Therefore, as you alluded to abaove, I use them to fill in forms in which most people use ballpoints. The spaces in the forms are typically small, as you probably know. In addition, during my work day, I typically write a lot of notes on a 3" x 5" index card. Because it is jam-packed with data, my writing will need to be fairly small. Hence, the fine nibs.
Even when I am writing at home, my penmanship reflects the constraints of my work writing, so I will tend to write smaller.
I therefore have to assume that most of the MB Medium nib fans must write in a larger script than I do.
Attached is a scan of a 3" x 5" card where I made some notes I would typically do (scan is magnified, so have to imagine it as 3" x 5")
Edited by kenny, 20 April 2016 - 05:20.