many "high-end" lines have a plus-size component. michael kors, ralph lauren, anne klein, etc. i don't necessarily want to see plus-size people in a victoria's secret runway
fashion shows, but i think it's good to have models in a range of sizes display the clothes IF your line carries a range of sizes.
there is zero reason why there needs to be a separation between plus-size fashion and high-end fashion. that being said, obviously not every look will work on a plus-sized body, but that's the point of having a plus-sized model show you what the clothes look like on her body.
for the record, though - most plus-size models look nothing like the average "plus-size" consumer in terms of body composition. have you ever seen the "controversial" lane bryant commercial with the attractive brunette in her underwear? she had a flat stomach! she's probably a size 10-12, and isn't a total slob. she's just a larger woman. the way she looks in clothes isn't necessarily representative of the way other plus-size bodies will look in the same items, but i suppose that's true of regular size women and their corresponding models as well.