There are two absolutely crucial requirements for shoulders in the swing...

1. How to use shoulders to generate power. 
2. How to keep the front shoulder in while coiling.

To tackle the first of these, let's start with a trend that seems to be showing up more often in recent years - not just loadng the arms to the back side, but loading the shoulders upwards.
What you'll notice is batters lifting their hands well above the head - almost like a pitcher in full wind-up. This isn't for style points... it recruits the deltoids, trapezoids, pectorals, and scapular muscle groups - the entire shoulder region. Some hitters find this helps to develop a more powerful loading action. If nothing else, it serves to stretch and loosen the upper body, and for that alone - freeing up the muslce groups - it contributes to the efficient transfer of power during the swing. We don't always teach it; we don't insist on it, but it has merit.
Details on coiling and a series of graphic animations available to Team Player members. Please log in or join.