Well, it's pretty much similar to having a Google account, and since you can disable what you want to sync or not to sync, it's not bad to just activate one.
For starters, you'll need one if you want to use Samsung's Find My Phone service as well as if you want to use their new Cloud service (which does help a lot to keep apps and settings backed up...just remember, app DATA isn't synced unless the app itself supports cloud saves).
I just use it for those two purposes (I don't sync my Gallery, contacts, or calendar because I already have those in Google) and because Samsung Notes (which SUCKS compared to the now-defunct S-Note) also uses that to sync notes to the cloud; Evernote sync is no longer native to the app.