The smarter multiprocessing for rendering by itself is a good reason to upgrade.  Indirect lighting has some problems but I'm sure these will improve as time passes.  I think the new rigging features have a lot of potential also.  The other new features (content library improvements, interface flexiblity) are things that some people will like and some people won't care about, but overall it's a very big set of improvements.

Installing the Poser 8 upgrade does not make your Poser 7 install unusable, but I didn't read the wording of the license all that closely.  In a practical sense, both can be used - I don't know if there is specific language about exactly what you can do with the Poser 7 license though (e.g. I doubt you could ethically sell it).
edit: actually the license explicitly says it is non-transferrable (can't be resold).  I don't know if this is enforcible,  There doesn't seem to be any other specific language about the upgrade in the EULA.

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