DNL DRM looks like it's an "online only" form of DRM--requires a constant connection. That means it won't work with many ereaders, and it won't work on some platforms. The site it links to, desktopauthor.com, says it works on Windows, Mac and Android... not Linux. And it doesn't seem to work on any e-ink readers.
Authors who believe their customers are all potential thieves who are stopped only by threats and physical restrictions are, of course, welcome to use DRM. It's not a "must" for any author who believes their customers are interested in supporting their career. Saying "I MUST lock this book up because otherwise, you're probably going to steal from me" is not going to build customer loyalty.
Every pirated copy is truly a dagger on their side.
No, the pirated copy may be a test run to confirm that the reader wants to read that book. Or it may be a book that can be read on their reader, because the author who chooses "must be online to read this book" DRM is saying very clearly, "people without high-speed internet, and people using Linux, are not my customers at all." In that case, those pirated copies aren't costing the author anything... the author didn't want to sell to them in the first place.
There are several kinds of "effective" must-be-online forms of DRM. None of them, you may note, are popular. None of them are used for bestsellers. Their largest market segment is college students who will pay $70 for a limited-use ebook instead of $120 or $180 for a print textbook... but there's also a huge underground collection of free PDFs of those textbooks.
Insisting that the only wanted customers are those who have certain devices, or read under certain conditions, is a great way to *stay* an obscure author in a niche market.