With Rapid-Ask by STS we give potential clients access to Rapid-Ask on our servers. Obviously some of them have privacy concerns. Up until now I hadn't even considered running multiple instances of SSRS on a server (we do this all the time for IIS and SQL Server for testing, but never even thought SSRS would do it).  What we used to do was to spin off a VM for each one but this is killing us on disk space and CPU.

Well, after a quick google this is what we will now do:

You can install multiple instances of Reporting Services on the same computer. Installing multiple instances is useful if you want to host different content for specific sites. Each instance will have its own report server database, configuration files, and virtual directories.

See Microsoft Technet for full details.