I've got one colleague who mistakenly let Dropbox psuedo-merge his personal and business accounts several years ago. He still hasn't completely fixed it!
I was around when they introduced the "multi account" support, and it was very easy to see how/why they did it. Instead of updating their clients to support multiple accounts (which they should have done), the client just keeps supporting only one account. Then on their back end they return "merged" content to the client. You can easily see how this can create chaos, because other people can also be updating your personal and business content, and untangling it all is a mess.
I was around when they introduced the "multi account" support, and it was very easy to see how/why they did it. Instead of updating their clients to support multiple accounts (which they should have done), the client just keeps supporting only one account. Then on their back end they return "merged" content to the client. You can easily see how this can create chaos, because other people can also be updating your personal and business content, and untangling it all is a mess.