Technically speaking, I should start by pointing out that Doctor Who books aren't actually gone. In fact, there's something like five lines of Doctor Who fiction out there--the basic novels, which have been a bit dormant but will see three new releases in April; a line of fancy hardcovers that have attracted jaw-droppingly good authors like Stephen Baxter and Michael Moorcock (I'm still holding out hope for a Harlan Ellison entry in this series, although I know it's not going to happen); a line of "Quick Reads" designed to be finished over a lunch-break; a kid's series of 2-in-1 novels; and a Choose Your Own Adventure-esque run of books. After all that, I can kind of understand why the BBC doesn't want a line of novels for the classic series cluttering up the works.
Nonetheless, I miss the books that were coming out before the TV series relaunched in 2005. Even though there was a fairly steep dip in quality during the changeover from Virgin to the BBC, the book series had recovered reasonably well by the end, and produced some really excellent work like 'The Tomorrow Windows', 'Camera Obscura' and 'Fear Itself'. It's a line that deserved to continue on its own merits and on the merits of its sales...but there are also three other reasons I'd bring back the Past Doctor Adventures book line.
1) There is a place for a line of Doctor Who books for older fans. I realize that this is a very fine line to walk, because Doctor Who is a family series and I don't want to see younger fans excluded...but at the same time, the genie's kind of out of the bottle, here. For a good fifteen years, the Doctor Who series was written with an eye towards the older fan, and we got to see stories written for a more mature reader...and I don't necessarily mean that in terms of sex and violence, either. 'Love and War', to choose a particularly excellent and seminal example, examines the Doctor's relationship with his companions and his ultimately alien perspective on the universe in a way that the TV series will never be able to do, simply because I don't think the TV series is willing to risk that kind of unsympathetic view of the Doctor. Not every book was that good or that mature (I'm looking at you, Chris Bulis) but there was a potential there that shouldn't be discarded.
2) The book line served as a laboratory for improving the series. Because the book line was for older fans, and because it wasn't under the pressure of being a flagship show on Saturday nights, they had a lot of license to experiment. The book lines came up with a number of interesting ideas, like a time-travelling archaeologist, or a human/Time Lord hybrid able to deal with the Doctor on his own level, or a view of the Doctor as a myth scattered throughout human history, or a Time War that would lead to the destruction of Gallifrey and the end of the Time Lords, or...basically, re-reading the books (like I'm doing here alongside my wife, just as a reminder) shows just how much of the concepts that became essential to the success of the new series came out of people trying new takes on a classic series and seeing what worked. That's the kind of thing that can and should happen again.
3) The book line gave a lot of excellent writers their first break. The Doctor Who book line had an open submissions policy, both in the Virgin and BBC era, and a lot of fans made the jump to pro through the book line. Paul Cornell, Mark Gatiss, Gareth Roberts, Gary Russell (whatever you may think of his work) and Matt Jones all went from fan to novelist to TV writer, and even some of the already established TV writers (like Moffat and RTD) started their work for Doctor Who in the novels. It didn't always work--we got the occasional Neil Penswick--but it really encouraged a lot of talented people and gave them an opening, which is something that I think feels appropriate for the BBC to do.
There are more reasons, some of which really can't be done right now due to the narrative primacy of the TV show...but I think there's a place for a line of well-written adult novels in Doctor Who, even now. I just don't know if the BBC will see it my way anytime soon.