A point or 2:
"I assume that the big reason to keep an online tree and sync it would be so that it can be part of the "hint system""
You get the "hint system" on FTM whether you have an online tree or not. The online system sometimes gives different hints than FTM. 2 benefits of online tree - backup of your FTM tree and you can elect to share tree with other researchers who can contact you and enhance your research.