This has been the case for me for the last couple of years. I used to see promotions like this constantly (even going back to when 8 free GB was on offer instead of 50), and as a result I have a small collection of different file storage services that add up to an okay amount of storage, mostly due to the couple of 50 GB promotions.

The one thing to be careful about with Box is the limitations they put on access; The promotion I got some time ago for 50 GB has a file size limit of 100 MB and doesn't allow download of folders with a free account. This makes it impossible to store a few large files and inconvenient to store many small ones (though WebDAV access makes the latter less of an issue.