Actually, this is just one of the many reasons why I love Aperture. Non-destructive, interactive image adjustments. I don't use adjustments very often, but when I do, they're absolutely amazing. I had to hunt through my library to find images that had many adjustments made to them - most of my images are used as they were taken in-camera, with only minor RAW tweaks. But, occasionally, an image needs some extra love. Two recent examples are a lightning strike I was lucky enough to catch last night, and my son's team photo for his T-ball team.
The lightning strike was taken on the spur of the moment, without any prep or composition. I just fired off a bunch of shots, hoping to get lucky enough to catch some lightning. Exposure was off. Contrast was off. Tweaking a handful of sliders in the Aperture adjustments HUD turned the image at top left into the image at top right. Much more dramatic, but not unfaithful to what I saw. All I did was drop the exposure a bit, and bump up contrast. Easy enough, and because it is an interactive adjustment, it took maybe 20 seconds to do.
The other is my son's T-ball team photo, which was unfortunately shot with the sun as a backdrop. Way overexposed, with too little contrast. Again, some minor tweaks of the Highlights and Shadows tool, and I got an almost usable image.
The best part is, all adjustments are nondestructive. I can easily modify any of the adjustments without having to futz about with file management or layers. I can toggle exposure adjustment to see how it affects contrast or white balance. I can easily toggle between unmodified Master and tweaked Version views to see the difference easily. Very cool. This one took me quite a bit longer - maybe 2 minutes - before I was happy enough (or realized that was as good as it would get).
I don't use it a lot, but when I do, I sure appreciate the nondestructive adjustments. Although I shoot RAW almost exclusively, these nondestructive adjustments work just fine for any file format Aperture can read, including JPEG. You get less data to work with in JPEG, but you can still take advantage of the adjustments.