My guess is that it depends on your preference and the size of the databases.
Appvolumes doesn't grow to extreme proportions and the composer doesn't as well.
Event DB and VCenter DB do have a tendency to grow to lareg proportions.
Still all in all we have our DB's for the VDI enviornment on 1 SQL server.
I would suggest to use a centralized SQL DB for Appvolumes though, easier is management with backups and multiple Loadbalanced Appvolumes servers.
