Is an emitter follower usually used with an AC source? I was doing some reading on it, and on most of the examples the base was AC. I can't really see it being that useful with a DC input.

 

An emitter follower is used with an AC input riding on a DC bias voltage or with a DC input. It has no voltage gain and has a high input impedance and a fairly low output impedance. Opamps usually have emitter-follower inputs.

 

This is an alternate use for an emitter follower...

It really depends on your imagination, it has some very useful characteristics.

 

One common application of an emitter follower with 'DC' is the output of a voltage regulator.

Look at the internal diagram of a 7805 regulator, the output device is an emitter follower. (That's why they are not 'low dropout', the input voltage has to be high enough above the output to maintain the base drive).

(Yes, PSUs have many possible configurations but the 78xx series are something most electronics enthusiasts are familiar with).

 

In the long run every voltage is varying.