One of the reasons I believe that the Watchmen collection has been a perennial best-seller for over twenty years is the fact that it is complete in one volume. An interested reader can pick up the entire story at once and not have to worry about finding Watchmen Book 2.
But for some reason, DC has been releasing their collections of their limited series in multi-volume chunks. This week sees the release of Rann/Thanagar Holy War, vol. 1, which collects just issues #1-4 (of 8) and the attendant Hawkman Special. The War That Time Forgot, a twelve-issue series, is being released as two separate volumes. And possibly the worst offender of all, Justice, the twelve-issue series from Alex Ross & co., has been released as three separate hardcover volumes (later as paperbacks), each collecting just four issues of the series! If you want the entire series in one volume, your only choice is the $100 Absolute edition.
While there may be some short-term gain for DC in releasing their collections this way, in the long term I think they'd be much better served by releasing an entire limited series together in one volume. Much easier for stores to stock, and much more attractive for readers to get the complete story in one purchase.