The previews in Aperture are very cool, enabling easy sharing with iLife and the rest of your Mac. And there's ample information on the web about how previews operate, my favorite being Joseph Linaschke's writeup on ApertureExpert - it's a great read. I use previews when sharing photos to Google+.

However, as your Aperture library grows, you may find that there are fewer and fewer of your photos you need previews for. When's the last time you needed to share a snap from that weekend roadtrip from last year? Or a child's 3rd birthday party...and your child is in college now? Or the EV-4 exposure in a bracketed sequence of shots? There's probably lots of previews in your library you don't need and are simply wasting disk space.

I consider my library small by comparison, but am well past the point of not needing previews for all the images in my library. I was curious about how much disk space previews were taking, so I disabled previews and deleted them all. The results:

A ~28% reduction! And my previews were not full size or maximum quality. Expect more space savings if you're generating large previews.

So saving disk space - that's the "why". And not what prompted me to write this post. I found it was slightly less than intuitive to delete all of the previews in my library. The video shows me going through all the steps, and here's the summary:

• To turn off preview generation...
  • In Aperture Preferences, click Previews and deselect 'New projects automatically generate previews'. 
  • In the inspector, click the Library tab and select Projects. Click the gear icon and deselect 'Maintain Previews for All Projects'.
• To delete existing previews...
  • Click Photos and clear any filters in place.
  • From the menus, do Stacks -> Open All Stacks.
  • Next, ⌘A to select all the photos.
  • Finally, do Photos -> Delete Previews

There is more preview control at a project level, and the video shows a little of that. Also, these steps are per-library. If you maintain multiple Aperture libraries, you must repeat the above steps for each library you want previews to be deleted from.

So what are the caveats? There are some, but most with a reasonable alternative approach.

• What about sharing with iLife and other apps? You are right if you're saying to yourself that without a preview, sharing from Aperture via drag and drop or the Media Browser doesn't work. It doesn't. But, there's a reasonable alternative. Generate an on-demand preview. At any time, you can select a photo and do Photo -> Update Preview. I use this method when sharing to Google+. (And this is a good one for a keyboard shortcut.)
• What about iPad syncing? Same as iLife sharing. If there's no preview, there's nothing to share to the iPad. This point is more painful for me. To date, my iPad syncing is based on Smart Albums. Unfortunately, Aperture doesn't offer the ability to maintain previews on a per-album basis. What I've been doing now is selecting all photos in an album and manually update the previews. I only sync a few albums, so its tolerable for me. Using Shared Photo Streams might be another viable alternative. But I believe this is only on iOS 6, which isn't supported on the original iPad.
• What about Flickr? Or Facebook? I'm not a Facebook user, but did try out sharing to Flickr. Even without a preview, a share to Flickr worked fine. Evidently, the flow for sharing to external sources doesn't use the preview....maybe it's akin to an export flow.
• What about PhotoStream? Works without previews. Again, I suppose the PhotoStream integration is more akin to an image export.
• What about my screen saver? I, too, used Aperture to feed my screen saver. Now that I've deleted previews, I export the photos I want to a 'Screen Saver' folder and point the screen saver at that.

For the most part, generating an on-demand preview is a workable alternative to unilaterally enabling previews. Where this falls short is when albums or smart albums are used for sharing. Hopefully, a future Aperture update will offer a "Maintain Previews for Album" feature. Add that to my Aperture 4 wish list.

Disabling and deleting previews isn't for everyone. There's certainly a trade-off between convenience and disk space. Depending on how you use your Aperture library and how much intra-app sharing you do, you may want to consider deleting your previews and generating only those you really need. I could see a workflow where one keeps the 'New projects automatically generate previews' selected, then every few months deletes all previews. The idea being sharing happens soon after photos were taken, and less so as the days and weeks pass.

Now go forth and reclaim some disk space!