iOS had multitasking since day one. How do you imagine iPhone was able to get call when iPod app was playing?
It was just not available for the third party apps, that's all.
In reality, Apple's implementation consciously limits what apps can do in the background to stop apps from draining battery life or generally doing things that the user is unaware of.
They deliberately limited functionality to provide a feature. They then provided more functionality which removed some of those original limitations.
Power efficiency has nothing to do with it in the slightest.
It's a lot harder to leave out features than adding. You have to be carefull about what you leave out, make sure you don't touch core-functionality, and make sure the features that are present are done brilliantly, because they will be in the spotlight, all the time. I'm not saying Apple does this flawlessly, I'm not a blind fanboy, they have serious misses (notifications pre-IOS5, Calendar app, IOS folders, ...) - but they do this a lot better than anyone else.
To be honest, I never had problems with "missing" features (except calendar/call overview in the lock screen), but more problems with the implementation of features that were present, but not executed well enough.
Also, in your sense every phone on the planet had multitasking because I can speak and listen at the same time.it was obvious that I was talking about multitasking for apps
So in the end everyone seems to agree that Apple, although easily possible, didn't want to give multitasking to its customers. Sounds even worse to me..