In the Advance Guide to Lightroom 4 video Jeff says he argued for Softproofing to be implemented as Snapshots instead of Virtual Copies.
This surely is not as elegant as Jeff argued for, but it is easy to use snapshots for softproofing instead of virtual copies if you want.
Snapshots, even snapshots of virtual copies, are stored in the file’s xmp space. In the case of virtual copies, the snapshot is stored in the master file's xmp (surprise!). So simply save a snapshot of the adjusted softproof. It is then available in the master file. Retaining the virtual copy is not necessary.
When you start to make a snapshot adjustment and receive the popup, if you select “Make This a Proof” and then create a snapshot, LR will name the snapshot as the proof profile (e.g., SPR3000 Velvet Fine Art Paper).
You can then have multiple snapshots in your master file for multiple output printers/papers. Much like we used to do with layer groups in PS for various output profiles.
If necessary it is easy to set any given snapshot or history state as the “before” (right click…).
I like snapshots since they are stored in the xmp space as well as the catalog.
I normally name my snapshots as 01, 02, 03 and so on, sometimes with descriptive text appended. Since LR displays them alphabetically, you can have them ordered as a chronological history.
Of course, this technique can be used instead of virtual copies in any situation you choose.