Apparently, with the lipo tool, you can still support iOS below version 4.3 and Apple won't reject it.
Google has been focused on enabling backwards compatibility since at least Android 1.5 (well before anyone knew how Android updates would play out in practice): http://android-developers.blogspot.sg/2009/04/backward-compa...
Meanwhile, Apple seems to prefer to make achieving backwards compatibility as hard as possible (short of explicitly banning it).
Still it is a hack that Apple could start disallowing at any time.
Also many libraries make use of lipo to create convenient static binaries that can be used both in the simulator as well as on the device.