The problem is if me and Joe use Dropbox for personal use, and then we come up with a great startup idea and want to start hacking on it. On like day 5 of working on this idea, you'll want to start sharing/saving some PDFs or excel sheets or something. Ideally you'd be able to create a new Dropbox for this project and run two clients on your machine, one for your personal Dropbox and one for your new project Dropbox, and Joe can do the same.

But because you can't run two clients at once, you can't do this. You can create "Personal" and "Project" folders in your existing Dropbox and sync to different machines, but Joe now needs to use your Dropbox account and he can't sync his personal account. Or you both use the web client, except you're basically using FreeDrive from 1999. Or you use an entirely different file sharing service like ZumoDrive, which is kind of silly.

I get that multi-client support for Dropbox is probably a niche feature and will probably lead to jerks like me using 5 accounts instead of 1, so they don't do it. And there are workarounds like Dropboxen that allow me to sort of have multiple Dropbox instances running on my machine if I have multiple user accounts. But man it'd be nice if multiple Dropbox instances running was a native feature instead of having the same, "Ok, I filled out the Articles of Organization and put them in my Dropbox so just refer to our filing number from there."; "Why don't you put them in MY Dropbox and YOU just refer it from there?" conversation with Joe.