From my point of view the point of doing a pre-release was noting that the books stuck on the DP queues are pretty darned good, that there are enough books stuck on the queues to represent about an addition 10,000 books to the PG corpus, that the books stuck on queues represent at any point in time about 1/3 of all the books ever started at DP, and that on average books get stuck on DP queues for about three years nowadays.
The idea was to get much of this "good work but not yet done" in front of PG readers, or readership in general, in order to reduce the amount of good volunteer effort lost by being stuck on the queue -- ie something like 1/3 of the all volunteer effort done EVER at any point in time is stuck on queue. The only way to get this "good work but not yet done" stuck in front of readers is to put it someplace they can find it and read it. The "obvious" place to put it would be in the PG database where it would show up with a "DANGER WILL ROBINSON" (or the PD version of such a message) in the database to the effect that this is a work in progress and is not at that state of completion that PG normally posts its work. The other way one might stick work in front of readers so that they actually can read it would be to have the work findable via google search, for example, ie roboted. Neither of these statements are true, so the DP reading room concept puts books in progress in a location that readers will not find those works in progress, at which point in time posting them to the reading room does not represent a contribution to the world, and the problem remains the same, namely: The books stuck on the DP queues are pretty darned good, that there are enough books stuck on the queues to represent about an addition 10,000 books to the PG corpus, that the books stuck on queues represent at any point in time about 1/3 of all the books ever started at DP, and that on average books get stuck on DP queues for an average of three years nowadays. Another way of saying this is that the other alternative is for readers to get the books from Google Books (assuming one can find them there) at which point in time the reader has a choice: a) get the Google PDF version of the bitmap photocopy of the book, which may or may not work depending on what one is using as a reader device, or b) get the EPUB or TXT from Google which is an uncorrected OCR of the book typically inferior in quality to that submitted to DP by content providers in the first place prior to round P1. IE even output from the P1 round of DP represents a significant contribution to many readers for whom the alternative is to try to read the unmotivated and uncorrected OCR output of Google -- but only if DP were to put that output in some place where real world readers can actually find that contribution and read it. Right now we still have the dogs guarding the straw.