You need to install propietary software to use it.

I use Dropbox as a glorified large email attachment distribution system, touching only the web interface.

Edit: obviously not clear enough, I'll try again: Is it true that 'You need to install propietary software to use it'? No.

I was wrong, and you were right. You don't need to install it but you will probably want it if you to take advantage of the full potential of Dropbox.

The web interface is proprietary too.

Which is largely irrelevant to the claim of "forced proprietary software install."

I don't get what you mean, but you can use their API to make your own front end.

I've never done that, I just use the browser.