@davidR: they want to keep the settings separate for one reason, be able to just populate the cache and not use it with your next full scan, meaning a deeper scan for security concerns. This makes the following full scans longer but no big deal as you won't run a full scan everyday, while shorter scans like the quickscan are set(at default) to use and not create, making them as fast as possible and using the job achieved by the full scans.
Okay this said I don't follow these default rules as I described it.
ps: following full scans, if respecting the default settings, will still be shorter because even if they won't use the persistence cache that they created previously, they don't have to create it again.