From the EULA: You may use one copy of the Software activated by a license key on a single device owned, leased, or otherwise controlled by you, at a single time (the "Authorized Device")
This appears to indicate that you may use only ONE copy at a time therefore it seems legitimate to be able to install multiple copies but USE only ONE copy at any one time. I suppose it depends on the strict interpretation of the complete sentence.
Personally, if Apple can sell family packs (5 devices) of software I don't see why other software providers don't do the same. Many households now have multiple devices, usually a desktop / laptop / mobile device but most do not use all simultaneously.