I don't agree. if you consider a transistor to be "perfect" with an Re connected in series with the emitter. then in a differential amplifier as the two emitter currents are anti phase the voltage at the connection will be zero AC volts, which is the same as when using one transistor and decoupling the emitter. In a single ended amplifier, the input impedance is Hfe time the emitter resistor or collector impedance of the current source so the input RC time constant increases.
Frank