If (hypothetically) your iMac originally had Leopard, and then Snow was installed, that still would not make that Previous Systems folder. That would ONLY happen with a Leopard install, or older. A Snow Leopard install, regardless of how or when it was done, does not create that folder. In your case, that folder comes ONLY from an old backup, and (because you can't install Leopard) that folder would not be created on your iMac as part of a system install (won't happen)....
But, to answer your questions, paraphrasing: (Are there any possible disadvantages to upgrading, instead of a full erase/install of your system?) Yes, there always can be issues, with unknown effects when the system is updated, rather than clean installed.
Most experienced Mac users will say that a good, clean install is preferable to an update install.
But, your iMac did not originally have Leopard installed, so your question is really just academic.
All that has happened is that you unintentionally copied a few apps that were previously part of a system on an older Mac. I think it would be worth your time to go through all of your apps (all of them), and make sure that you don't have third-party apps that are marginal on Snow Leopard, and your intel processor. Even if you chose to do a completely clean install of your Snow Leopard, you would want to make sure about any apps that you choose to install after that, correct? It will take some time (maybe a couple of days) if you have a lot of added apps and utilities, but could be worth the time, moving forward. ** get rid of that Previous Systems folder, if you haven't done that yet. Might be the main place that you'll find apps that you don't want to keep now. **
Check at the web sites for individual apps that you might have questions about, or you can certainly come back here with questions about any apps. Someone here should be able to find out for sure.