well,
please don't forget that a proper database configuration does keep it's transaction log files on a physical volume other than the database files.
If not doing so, you will never be able to run a complete database recovery (restore last online backup and recover transaction log until the crash) after a disk crash. If the transaction log (point in time recovery) information is stored on the same volume as the database it will be lost together with the database in case of a disk crash.
regards
Peter
PS: for test systems it may be ok to put everything on one or two volumes if IO bandwith and recovery questions are of NO interest.