The idea is that the spine crawler could only detect ground the spore crawler could only detect air.
The spore crawler feels by far the best static defense in the game. While the spine is usually only used for cheese. By splitting up the detection, the spine gains more utility and the spore is more in line with other static defense options in the game.
Currently Spore Crawler provides protection* vs Dark Templars, Ghosts, Oracles, Banshees, Mutalisks.
Protoss & Terran already have great early game harass options and I see no reason to nerf Zerg in that regard.
*The only thing Zerg needs vs ground stealth is detection and to roll target over with zerglings, queens and whatever is on hand.
What other static defense can detect only air? 
Why can’t creep detect invisible ground units? Or something similar to Terrans sensor tower. That would be yuuuge.
Might as well just ask for it to be like broodwar, and have all overlords just start as detectors