Hmmm... might have to fix that. Duck and I placed lots of enderman spawners all over and under the End but we hid them so they weren't as obvious. The aim was to make you deal with the Endermen, not the spawners.
Originally Posted by http://www.minecraftwiki.net/wiki/Spawner
A monster spawner is only active when a player is within a radius of 16 blocks from the spawner. While the block is active, it will spawn mobs within an 8 × 3 × 8 block area (8 wide and 3 high and 8 long), centered on the corner of the spawner block with the lower x and z coordinates (i.e. the northwestern corner). Mobs can spawn anywhere in this range that is suitable, even if the immediate vicinity of the spawner block itself is not suitable. The horizontal distribution is nonlinear, with positions near the spawner more likely than ones farther away
Therefore, you needed to get near enough to the spawner to make it spawn endermen (hence why we placed lots of them). Consider them an on-course hazard now for the End.