Where are small-signal differential amplifiers used?
A similar circuit is on the input of operational amplifiers.  Yes-- it saturates quite easily, which is why it is usually configured in a negative feedback arrangement.

In fact, Analog Devices uses almost exactly this circuit in a document describing how to protect input stages on opamps.  Real op amps are much more complicated, but its a good first order approximation.

 from http://www.analog.com/library/analogDialogue/archives/41-05/input_protection.html

Look familiar??

Notice, the output of the circuit is fed back to negative input, introducing negative feedback and keeping the overall gain in control.