Your thinking is correct. Sync can be considered a form of backup. There are plusses and minuses to using full, bi-directional sync to "backup" your data. Since we don't have backup available yet (it is coming), there are users that use "sync to odrive" for their own backup use case.
Sync is a much heavier process than pure backup, so when you apply sync to more massive data repositories, like multi-terabyte external drives, or NAS, it can make odrive work really hard to continually ensure that both sides are always consistent. This can create some CPU heat, large memory footprints, and excessive network calls. Sync also needs to be much more aggressive than traditional backup, since a key component of a good sync engine is speed of reflection on both sides. In a traditional backup use case much of this sync work is wasted, because the user is never going to change anything on the remote side and local changes do not need to be reflected as quickly as possible. Additionally, sync has the potentially to push deletes to the remote, which is not something you generally want for a traditional, pure backup use case. Of course, we have odrive trash as a safeguard against unwanted deletes, but, as you noted above, if would be a pain to have those pending deletes hanging around in the odrive trash, forevermore.
In my case, I use "sync to odrive" on the important folders on my local systems that are heavily used, as a "working backup". Folders like: Documents, Desktop, Downloads, Pictures, Music, and Videos. The names may very slightly between OSs, but I have these default OS folders all mapped to corresponding locations in Amazon Drive.
Generally speaking, most of the files I work with on a day-to-day basis are in these locations. Mapping things in this way allows me to "backup", without me having to change any default behavior. It creates a cloud-merged view of these folders across all of my systems. Everything I touch, on any system, is automatically backed-up to the remote storage and then synced down to my other systems as placeholders or locally cached files, depending on the location and use case.
Since many applications like to default to these locations, it gives me the advantages of cloud backing (backup and multi-system mirroring) without having to configure these applications to use non-default paths. With this type of setup I also have the flexibility to move any placeholders to any other locations that are mapped. So I can move a placeholder from my Downloads folder to my Documents folder or to my Desktop. This move is reflected in the cloud and across all of my devices.
Long story short (too late), my recommendation is to wait for backup for your larger data repositories that need the more traditional backup treatment, where you are not interested in two-way reflection and want to remove any possibility of local deletes. For datasets that fall outside of that, "sync to odrive" can be an extremely useful form of "backup", and a feature that I have found to be fundamentally transformative in my own use of the my local systems, and cloud.