Fashion and advertising and landscape photographers have been mentioned. But anyone whose work will be printed LARGE wants something other than a little dSLR. See this article
Pentax 645D -- First Impressions & Interview and note that at least one of the display prints looks to be a little under 2x3M (6.5x10 FEET) in size. And it's being looked at closely.
Anyone shooting photorealistic fine arts and/or posters, anything for big close-up display, needs larger formats. More than a few people still shoot medium- and large-format film, and some publications prefer or require such, because digital still can't match the resolution.
No, FF and MF and LF are NOT required for NON-photorealistic large prints. Some stuff I shot with a tiny 1mpx P&S (912x1216 pixels) has been blown up to 18x24 inches, and sold. But they were obviously NOT detailed images -- they were high-contrast and posterized and un-subtle, some were semi-abstracts, and they worked on those terms. But they were like cartoons, compared to a fine engraving. Some work just demands higher-than-consumer-level resolution.