All of the accounts I use are paid. It seems rather stupid to harm your paying customers the most! They could have solved the multiple free account issue by only allowing one free account logged in at a time.

It certainly seems stupid to mishandle someone with multiple paid accounts. I, on the other hand, have a pretty large free account with Dropbox which I acquired by evangelizing them, and have had various paid accounts from companies I worked for. Do you think it makes more sense to harm me?

Presumably their concern is someone creating 10 free accounts, and pooling/using them simultaneously. If their software and website (at the same time) supported any number of paid accounts with at most one free account then neither you nor I would be harmed.

I think I have about 40GB of free storage with DB as a result of working there referral system. However, a friend bought some Android based phone, came with DB pre-installed, and he has something like 500GB or free storage. They were doing some promotion.

I think one of the main issues with DB is that it is too little space and too much cost to add more. Considering what they pay for bandwidth, any paid plan really should be unlimited.

Dropbox does support a paid business account and a personal account. The paid business account just needs to be a Dropbox for Business account.

The issue is that it doesn't support these simultaneously. While you can do this pseudo-merge of a personal account into a business account, it is clunky, problematic to undo, and only helps with exactly one business account and one personal account. If you have more of either than tough luck.