Where are the zsync files for the CD-sized ISOs (such as desktop-i386) for the Precise Beta 1? They don't seem to be in the expected location at http://cdimage.ubuntu.com/releases/12.04/beta-1/
The betas are "released", you'll find them where you'd find any other release of Ubuntu: http://releases.ubuntu.com/12.04/
For ISOs that can be regularly updated then downloading from a zsync is a good idea.
The daily builds area (link below) contains a snapshot from the automatic build area - no obvious quality control other than it just builds.
For ISOs that are fixed at a certain point in time such as the beta then you should use the torrent file (preferably) or download direct via the iso itself. You can then use the normal update-manager updates to get the latest.
The beta zsync files are useful if you want to take the latest updates between beta releases without going through the update-manager route.
The area you have pointed to are for the less frequently used downloads - primarily the new DVD and ARM images.
For the desktop Beta downloads area the link is:
You'll note that this area contains the zsync files for the beta.