> > run the various computers
>
> Are you saying dropbox works without computers O_o?
I'm saying that Dropbox does not require you to run every computer yourself. If your house burns, floods, gets burgled, etc. not losing every copy is a really nice benefit for the average person. Not needing to make sure that their laptop is running at the same time as a desktop computer is similarly nice for being able to depend on having n > 1 copies.
> > provision enough storage to hold everything
>
> It's true that on all your clients put together, you need enough space to have one copy of your data; where for dropbox you can have one single sparse checkout and let their servers hold the full data set. I've never seen that be an issue in practice though - on the contrary, my laptops have enough space to hold all my data, and my dropbox account doesn't, which is why I went to syncthing in the first place.
It definitely depends on the user and is less of a problem as SSDs get larger but I definitely know people who had things like large photo/video libraries which they didn't want to have taking up 80% of storage on every computer they own.
> > secure them
>
> What security do you need to add to syncthing that you don't need for dropbox?
Two aspects: one is relatively low-impact but still worth making sure you're comfortable with any risk of data loss. If you use FDE on your phone / laptop but someone steals the desktop which has all of your synced data on it, is that a problem? Do you forensically wipe drives before getting rid of them? Dropbox's data is encrypted at rest and since they're hosted in a proper data center you don't have to worry about someone stealing a copy as easily as breaking into someone's apartment.
The other is more important now that ransomware is an industry: if you get malware, how robust are your recovery options? Simple versioning doesn't help if, say, the malware touches a file multiple times or if the versioning system wasn't designed to handle malice and so e.g. an attacker can just empty the trash or overwrite the old versions too.
One nice thing about the hosted model is that it has a completely different trust chain so even if you're totally compromised it doesn't allow them infrastructure-level access. That's far from perfect but enough people have recovered deleted Gmail messages, Dropbox files, etc. that it's worth asking whether you're comfortable about your data recovery options in any comparison.
> > make sure you have a backup plan
>
> I guess dropbox counts as having offsite backup implicitly; personally I'm using syncthing between a few laptops and desktops in different locations and counting that as the off-site backup plan.
That probably works for most scenarios other than a bad security compromise but how frequently do you verify those copies? Does syncthing have checksums to ensure that the copy you think you have hasn't been corrupted?
Again, I'm not saying that syncthing is a bad choice, only that “It works better” is a very broad claim which is clearly not true as a general statement. Convenience and reliability have significant value to most people.