Is a new Windows install better than an upgrade. Heck yea! Same is true with Outlook. Outlook leaves crap all over the place and the best way to make sure there are no cached files and resources is to remove and re-add a profile by following our KB. This will also solve the most common issues with Outlook: corruption, bad sync states and deep rooted issues in Outlook itself.
If you have three Outlooks connected to a single mailbox and only one Outlook is having an issue and you have access to that client... I would have them remove and re-add the profile so the other two Outlook clients and other clients (EAS, EWS, DAV) don't do unnecessary work.
"Resync all Devices" should be used:
when clients have are generally heathy;
when you want to data refresh all clients;
when you don't have access to the client and want to try to fix;
"Resync all Devices" shouldn't be used:
when your client is really messed up;
when you have a ton of clients connected to a mailbox with limited bandwidth;
when your concerned about server bandwidth;
when you have access to the client and can reset it on the client;
I can go on and on but I hope this helps.
And this applies to EAS, EWS and a variety of clients.
Don't let the connectivity list really mess with you, its becoming a bit obsolete as we introduce more functionality.