You can hold down the shift-key while lauching Aperture - this will defer the generation of previews. Then check your Aperture Preference Settings again, if you really disabled the generation of previews.
If you have a protection problem with the user preferences file, Aperture may not have been able to save your settings. In this case it should help to remove the preferences file "com.apple.Aperture.plist", see the paragraph "Delete the User Preferences" in :
Aperture 3: Troubleshooting Basics: http://support.apple.com/kb/HT3805
This will reset the Perferences to the default values, so you will have to set the Preferece Settings again. Make sure you launch Aperture with the shift-key hold down, until the preference settings are fixed.
If you really need tp have previews, will depend on wheter you have a rereferenced or a managed library. If your library is referenced, I strongly recommend to have previews, like Allen stated. With managed library previews are optional, but you will not be able to use your images in the Media Browser.