Actually, it's far from inevitable. Apple will likely be moving iMac to Arrandale processors next year, retiring current Penryn lineup. And even then we may only see dual-Core systems, not the quads. Arguably Arrandale will offer enough of a performance bump over Penryns, even in dual-Core configuration. But the quad-Core iMac is not guaranteed by any stretch of imagination.
And when the highest-end systems were the only one with dual cores, it wasn't inevitable that Apple would use them?
Once most apps are multi-threaded, more cores will make things faster. Some apps already have those benefits.
Only a few make single-core systems now. We're on to dual core now, and eventually on to quad core, then octo core, and so on.