Firstly I should point out that there may be some licencing issues around this if one taken advantage of an upgrade offer from an earlier version rather than purchased a later version licence outright. However I would guess (I don't know and I have not tried to check) that retention of earlier versions for support purposes within an organisation would not be considered unreasonable. The need could be expected to fade away over time.
There are some potential issues about what windows ends up with registered as the default settings locations in the registry (for example) and for the Help file. The help file does not seem to be a shared reference on my installation probably due to the New type of file for V9.
However the options settings - Folders and so on - are shared in the registry settings. For support purposes this is not normally a problem - but then I rarely use the default folders anyway.
However there are other settings stored in the Windows registry and whilst I have not to my knowledge experienced any problems related to them I can't claim to have covered that ground in any level of detail. The need has never arisen.
If you can live with those potential constraints then the versions seem to work fine installed in parallel with the usual caveats about backward compatibility.
Of course I do have the luxury of running my own independent system without the need to consider corporate infrastructure. I'm not sure if that would have an influence.