I would say the substantially improved glyph rendering for fonts highly functional for many people using Illustrator, also being able to jump up to higher resolutions quickly to minimise UI area is great...
You are less likely to get aliasing artifacts with complex illustrations when zoomed out, and will resolve substantially more detail in such cases.
You can see small details without having to zoom back and forth. If you're working for print it's more representative of how it will turn out. You'll be less likely to go "well **** I didn't notice this tiny detail that's annoying me". Stuff like very precise alignment.
Other than that the rMBP's display has very accurate colors and has close to 100% sRGB gamut which is a plus for designers, although that's not a result of HiDPI.