This is the simple low cost way but not always. Many Motorola phones have switchover capability between battery and charger sourcing. They can run with dead battery or even no battery installed at all.
Why is this a problem, do you mean? The internal circuits are not supplied directly from VBAT. There is always an LDO or a Buck between that regulates the voltage. Of course these are also internal circuits in the analog ASIC, but they can withstand a varying voltage input.
I would like to build a Impulse cell phone battery charger with USB output. It will be some kind universal charger. Input Voltage 230V - Output Voltage 5V. Is there any body who have design of this? I'm will be very glad