This is probably one of the easiest ways to extract files out of an MSI. Note that the installed "MSI MENU... has options to do this for you.
If you want the files uncompressed outside of cabinet files then an administrative install of the MSI MAKEMSI creates is the simplest way to generate the image.
msiexec.exe /a "C:\MSI\some.msi"
Note that the above will not extract files stored in the "Binary" table, the easiest way to extract these is to use the "export" function in "ORCA" (or see the "Export to .IDT" shell extension).
If you need to automate the process then something like the following is what you need:
msiexec.exe /a "C:\MSI\some.msi" TARGETDIR=C:\tmp\Some /qn