So, a 1 pixel transparent gif file embedded on a web page, served from a Dropbox for Business account would be a great way to gather a big list of names & email addresses of your website's visitors.
Yes, this was exactly my concern. Moreover, this would be segmented data of potentially higher-value visitors because we can presume that Dropbox Basic and Pro users are more sophisticated than your average internet user. E.g. they are more likely to be syncing data across various platforms and own and use smartphones and tablets.
There is an analogy here with LinkedIn, whereby you can opt in to the feature which allows you to see who has visited your profile. However, there is a very clear warning step where you need to agree to participate, and it's reciprocal, i.e. others will also see when you have viewed their profile. Moreover, this feature is available to both free and paying users. What Dropbox is doing is very hush-hush. It looks to me like they don't want Basic and Pro users to find out that their identity is being sold to Dropbox for Business users.