There aren't statistics that I know of. Though anecdotal evidence suggests that there's quite a lot of PDFs with this problem. There are a lot of people whom export to PDF because users then can't "edit" the file.

So far, coalescing works, but we've had to make substantial changes to a lot of places. The hard part is still ensuring it is compliant with PDF specs, which we're in the process of working through before we submit it as a patch to the PDF.js team.