It's important to keep in mind that what was fairly rare 15 years ago is not rare now. Melon was a bunch of stuff that would have been really expensive and rather challenging to track down separately. I think people have lost sight of how difficult it was to get a hold of all of the respective singles in 1996.*
True, true. The Melon tracklist in 1995 included:
Lemon (Perfecto Mix) - available on a limited-release single in US, Australia, and Japan, and on the "Stay" single
Salome (Zooromancer) - limited edition UK single of Who's Gonna Ride
Numb (Dignity remix) - wasn't available
Mysterious Ways (Perfecto) - limited edition UK/Aus single
Stay (Underdog) - wasn't available
Numb (Soul Assassins) - wasn't available
Mysterious Ways (Massive Attack) - wasn't available
Even Better than the Real Thing (Perfecto) - released commercially on the remix CD single
Lemon (Bad Yard Club) - released on the Lemon and Stay singles
So of the 9 tracks on Melon, four were completely new, and another three had very limited releases. Only two were widely available. Contrast this to Duals.
At the same time, the vaults are probably pretty bare at this point. Most everything from 80s U2 has been released, apart from some Rattle and Hum demos (Wild Irish Rose, She's a Mystery to Me being the most obvious).
And I'm not convinced there's a lot of quality stuff from 90s U2 we haven't heard either -- between the Achtung B-sides and demos, most everything from those sessions is out there in one form or another (some of it to the band's chagrin). Obviously we'd love to hear some of the Zooropa/Pop sessions, but I'm not sure there's missing jewels that we haven't already heard, and I'm not sure it's in the band's best interest to release rough demos of those years, particularly given that some of the songs recorded for Zooropa served as a jumping-off point for Pop (Velvet Dress, Wake Up Dead Man and God Will Send had their starts in those sessions), as well as the way Pop was recorded (looped drums that Larry re-learned later).
ATYCLB is a different animal -- there's apparently a wealth of good material, only some of which we've heard. I'm definitely curious to hear more from those sessions, as well as some of the material from HTDAAB, though I'm less certain that there's much left from the HTDAAB sessions, since a few of those songs had come from earlier albums ("Original of the Species" was originally a contender for ATYCLB, "City of Blinding Lights" had its gestation during Pop).
Still, U2's position regarding the fan club might be that they just released a bunch of "rare and unreleased" material two years ago, so there's no need to go further into the vaults -- and McGuiness told Propaganda back in 1991 that "we don't feel under obligation to release everything we do to the public and we don't view fans of the band as librarians who want access to everything we ever do."
So what we have may be what we get.