GhostofCain
Ancient Mariner
To me, the conventional wisdom that Steve Harris is the be-all and end-all behind Maiden's songwriting has always been absurd. He's a key contributor, but there's a reason that Iron Maiden, The X Factor, and Virtual XI often round out the bottom of people's album rankings (no Bruce, no Adrian), and Killers (no Bruce), No Prayer For The Dying (almost no Adrian), and Fear Of The Dark (no Adrian) are rarely near the top. Also, compare Bruce & Adrian's solo work to Iron Maiden's '90s output, and I think most fans would give the edge to Bruce & Adrian.
Although I would argue that Steve has written more great songs than the number you mention, I agree that the magic happens when you have Bruce, Adrian, and Steve contributing to the songwriting.