Why "one app per VE" deployment approach?

We have groups of apps on 1 physical server in hosting rack i.e. 3 web apps on server A talking to database on server B.
Physical staging servers sometimes become quasi-production servers. i.e. can we pull the plug on them without warning? (Note: This has already happened with one of our VE's :-)
It's nice if staging environments are real sandpits.
Reduced performance is usually ok for staging environments (and there will likely be plenty of resources most of the time). Not used for benchmarking.