I can't see any reason (at least not without more info) as to why they couldn't be under the same SQL instance. We use a consolidated SQL instance model (meaning multiple apps to 1 SQL server) and it works fine.
The disadvantages that I see are:
- A little more difficult to troubleshoot performance issues
- All your eggs in one basket (unless you cluster / mirror / log ship)
- One site sites DB usage "could" affect the others from a CPU / memory perspective.
The advantages that I see are:
- Easier to admin (backup for example only needs to be configured once)
- Less SQL servers to manage (sort of realted to 1)
As far as I can tell, the 1 app to 1 SQL server model is sort of going by way side. I'm bias and personally a big fan of multiple apps to one SQL server. Just don't skimp on the resources for that SQL server. That's not to say that you need to over spec, understand what performance needs the sites need. Also, as an FYI, all our SQL servers are virtual, incase anyone trys to throw that gotcha at you, we've had no problems. All comes down to proper sizing.