Much as I hate to admit it I'm afraid the pro DRM people have a point about casual sharing, you essentially have 2 different types of method to "Get a book for free that you were meant to pay for" (And let's see the symantics police argue with that definition)
1. Torrents, mass sharing sites etc
- I get the feeling that most people do not use these sites to avoid DRM, it is easier to learn how to strip DRM than it is to find the correct book you are after on a torrent site and download it. I suspect they use this method because they don't want to pay (Whether it's because they have no money, won't pay agency prices, like free stuff or any other reason is pretty irrelevant at this point), possibly some of these are lost sales (someone might pay for the book if they had no other choice) but, I suspect, in the main these people are not your customers, you can make the book DRM free and the bulk of people using torrents etc won't care, they will continue to torrent.
2. Casual sharers (Book groups, facebook groups, etc.)
- These are the people generally stopped by DRM (Not all of them, obviously, but a largish percent). In the main these are people who share books without realising that it is not "what the publisher intended" (Again carefully worded to stop the "ebooks should have the same rights as pbooks crowd"). These people probably are your customers (potentially at least) and may actually buy your book if a member of there group can't just give it to you, it stops word of mouth advertising to quite a large extent as they can't lend just one book as with a pbook.
On this basis (and I could be way off track here) DRM probably does create more sales.
This is of course affected by genre.
The staunchest anti-DRM folks are normally us techies. And as techies we are best placed to ignore the entire DRM problem anyway - Which would explain why MacMillan have made the science fiction based Tor imprint DRM-free, but have not done the same for any other imprint (No point, coupled with the image that all sci-fi reading techie types sit in darkened rooms and don't talk to anyone, making social network sharing a moot point
).
So until the public understands that they are not meant to share books around like paper books on steroids DRM will probably get you more paying readers in the short term.
In the long term you need a plan for what happens when people move platform / file type and want to re-read a book (again this will be more/less of an issue depending on genre).
So, to sum up
Short term - DRM good for stopping casual sharers (Probably giving a boost to sales)
Long term - Annoyed customers who have changed ereader platform (although from the author/publisher point of view the customer is much more likely to blame the shop), some/most? people may just see this the same as something like the VHS-DVD-BluRay constant upgrade path and may, grudgingly, be ok with it.
Long term - less readers from "word of mouth" sharing of books, a lot of people may be willing to read a book based on someone's recommendation if they hand them the book - but less will read it if they have to go out and buy it themselves. How much this can be offset by offering one of your books as permanently free from amazon/kobo/your site I don't know (although if you offer a permanent free book I would mention it on the also by page in your ebook)
Essentially I think going with DRM will be less harmfull (depending on your genre) than going without at the moment, but I would keep your ear to the ground and be ready to go DRM-free the moment it seems like the way to go. MacMillan will be a good publisher to watch as they will have the only real world large scale data about DRM v DRM-free soon, if the Tor imprint gets DRM'd again then go DRM if they expand the DRM-free policy to all imprints (or even just for an imprint that matches your genre) then go DRM-free.
It's an uncertain world, everything could eventually end up either DRM-free (like music) or DRM'd 3 ways to sunday (like video), but either way it will take a while to sort out.