Technically you're multitasking without double tapping. You can still go the home-screen-first route and the app still stays in the background. Double tapping home is just an app switcher.
So if I'm outside going somewhere and I have shopping bags in one hand, you think it makes sense that I have to put them down in order to do the three finger swipe, pick them up, and go on my way again? Doesn't that defeat the purpose of multitasking?
I'm glad you don't work for Apple...
See the above comment 🙂 The gestures should be adequate for MOST situations, but if you can't happen to use both hands... then just don't use the app switcher! Plus, I'm not too opposed to allowing the double tap as an option (I'd never use it but options are good), as long as they had an additional solution that still allows you to keep the double tap how you like it.