Well... to be honest, every breed was designed and bred for specific disciplines, jobs and temperaments to match - of course they're going to be stereotyped, because we made them to fit a specific 'style' otherwise we couldn't even group them into breeds.
The problem comes from people insulting particular breeds, but the truth is not everyone likes every 'style' horse. I'm partial to the heavy breeds myself and would prefer not to work with hot-heads, so I avoid Arabs, TBs, and other typically hotter horses. Not that I don't think they're perfectly lovely horses, but because they just aren't my cup of tea.
Most people I know are exactly opposite, they'll take a nice bubbling hot TB over my mellow mooshy drafts.
So truth is - people need to be picky about getting a horse of a breed that DOES suit them, they should avoid breeds that are not their style. This way there's less mis-matched horse ownership.
Just like I believe someone looking to reach the high levels of any competition style should buy a horse of a breed designed to buy it. While I believe any breed can compete low-moderate level in any competition, even if it isn't particularly fitting. But really it's best for the horses not to be pushed beyond their conformational or emotional abilities just to reach higher levels of competition.
So I don't see breed prejudice as such a bad thing, I only find it bad when people are jerks about what horses Other people are riding. It shouldn't matter a wink what horse someone else is riding, it's not yours ;)