When you are building tools that use WiX, it can be difficult to detect if the WiX toolset is installed or not because WiX uses a version number in the Registry tree as a key. For example, I just added support for building Orchestrator Integration Packs from workflows to a tool I was working on, and it leverages the WiX toolkit. The code I already wrote checks to see if WiX is installed by looking for HKLM:\Software\Wow6432Node\Microsoft\Windows Installer XML\3.5 and then it gets the InstallerRoot registry value to access the tools. Then when upgrading to 3.6 RC I broke that logic because it is dependent on a version number.
Please update WiX so that it writes the Registry values to a key that isn't a version number. This will make it much easier for anyone who is taking a dependency on WiX, allowing them to test for WiX reliably and across versions without having to know which version is installed first.