From 5.0 to 7.0, there really aren't that many reasons that I can think of. In fact, the code is almost the same - the 7x documentation is actually the 5x documentation. So, aside from product lifecycle - the biggest reason IMO is support for virtualizing Unity on VMWare in 7x. The second would be support for Exchange 2010. Only 7x supports that version of Exchange at this time.
As for 8.0, I wouldn't recommend going to any 8.0 releases at this time. There is way too much churn unless there is a feature you absolutely need to have. The real push these days is on migrating over to Unity Connection unless you have a hard requirement for UM. But for 8.0, there are some new features as usual with Unity but, again, IMO I don't see as much product development going there - more maintenance and preservation of product with some new features here and there but I dont know of any blockbusters.
Hailey
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