I don't think they had a choice. They were a year late on U2 by U2, so they had to fulfill that commitment to HarperCollins. Likewise, they had one album left on their contract with Island, so they probably figured they might as well get it over with.
Given the lead time in publishing, the contract for U2 by U2 was probably signed in 2002 or 2003, aiming for a 2005 pub date. That would have made sense at the time, given the cadence of Bomb in 2004 and the book in 2005 (with no thought to a hits album, i'm assuming,) Instead, everything got jammed together -- the end of the Vertigo tour, the book and the hits album.
I don't think the question is, should they have waited. I think it's, "is this too much U2 at once?" Certainly not for the faithful, but it runs the risk of overkill.