It's really a huge disappointment that Parallels doesn't seem to be able to offer a hassle free upgrade process to Windows 10. I would expect that they had enough lead time to thoroughly test this between the announcement and launch date. We shouldn't have to go through all the hoops that have been posted all over the forum to make this work! Also, the responses from support with links to KB items that clearly don't address the issues raised is another disappointment. Get your act together Parallels!
All the blame cannot be placed on Parallels in this instance, Microsloth deserves the lion'e share.
A very large percentage of the issues being encountered on this forum are also being posted
all over in forums with people using real PCs, including the display adapter issue even on new PCs.
The whole "reservation scheme" was hokey from the start. Basically their whole update process
is very buggy and even with all the time in the making, it still wasn't really ready for prime time!
As for me, I had clean installed Win10 on a VM on my iMac without a hitch, but took 3 attempts
to get installed on my MBP.
As to the OP's original question, if you have no interest in any of the new features in win10
and are happy with how Win7 is working, don't update. However, eventually you will need to
update to some new Windows OS in the future because Microsloth will be dropping Win7 support,
i.e. no security updates.