A case for having your math font be something other than italics|
If your math font is just the italic version of your text font, what do you do if you have a section of text aht you want to ital and that section of text contains math text? Making the math text roman just looks weird, and leaving it ital makes it so that it's hard to pick out from the text (think about just a variable by itself). So, what do you do? You use a different font for math. The book I'm working on uses a font called Euler, kind of a weird serify font. I hated it at first, until I discovered how nice it looks in the middle of italic text.