The Slide Step

Let me say right off the bat that I am not a big fan of the slide step.  In my opinion, it is way overused at the younger levels and even at the high school level.  I even think some arm injuries are at least partly caused by too much slide stepping.  I'm sure there are those who will disagree with me but I feel the slide step gives up too much to the batter and places too much importance on the runners.  It prevents many pitchers from getting to their release point on many if not all their pitches as well.  There are a number of other ways to hold runners close without have to slide step all the time.  Many were addressed HERE.  The slide step certainly has a place in pitching but it must be used sparingly and correctly in order for it to be most effective.  When I work with pitchers on this part of their game, I make sure to pass on the following:
Why?

Very few MLB pitchers slide step
because of the negative impact it
usually has on their pitches.(Photo by Jonathan Daniel/Getty Images)

Pitchers first need to understand the purpose of the slide step.  The main idea is that it gets the ball to the catcher much faster than a traditional, high leg kick.  This, of course, allows the pitcher to better control the running game since most runners will not be told to steal when a pitcher slide steps.  As long as the catcher is halfway decent at throwing to second base, the slide step virtually shuts down the straight steal.  But all this comes with a price.  Most pitchers who slide step, especially those who do it all the time from the stretch position, are up in the strike zone more.  This is especially true with their off-speed pitches.  This is due to the fact that the pitcher's arm is not given enough time to get down, up, and over to the proper release point.  Because of this, the pitcher's arm lags behind and releases the ball a bit too soon.  This brings the ball up in the strike zone making it easier to hit.  Many young pitchers also have a slightly slower arm speed on their off-speed pitches making it even more likely the arm will not get to the proper release point.  Curve balls hang and change-ups flatten out as a result.
There is a second (I believe more productive) reason for the slide step.  It can mess with the batter's timing. Continue reading to get more information about this aspect of the slide step.
When?
In my opinion, no more than 10% of the pitches thrown from the stretch should be of the slide step version.  Of course, this is not written in stone.  It's just my opinion of an estimate.  The point I'm making is that pitchers really have to use it wisely for it to be most effective.  A slide step should not be used in a non-steal situation - runners on 2nd and 3rd for example or any other time when a runner is obviously not going to steal.  It could be used when there is a base stealer on first and the pitcher generally wants to alter his times to home plate to avoid any patterns.  Another time might be following a big leg kick that baits the runner and his coach into thinking the pitcher is very slow to home.  However, I think the best time to throw it is when you want to mess with the batter's timing.  Hitters get used to the rhythm and timing of a pitcher's delivery.  When a good hitter seems to have a pitcher timed pretty well, a slide step can get him off the pitch a bit.  Do it once and the hitter will forever be conscious of you trying to quick pitch him.  That uncomfortable type of thinking is in the pitchers favor.  Be sure to come to a complete stop when you come set though.  I've seen many pitchers balk by not coming to a complete stop and trying to be too quick to home.
How?
As stated above, a big problem with the slide step is that it gives the pitcher less time to get his arm up to the proper release point.  This previous post HERE gave some pitching tips on where and how to come set with the glove to combat this.  An additional tip would be to come set with most of your weight on your back leg instead of evenly distributed among both feet.  A big part of pitching is getting your weight back before you go forward to throw.  Keeping your weight even and then just sliding the front foot forward to throw never gets the weight back.  This loses a lot of power for the pitcher because the pitcher is only using his arm and not much of the rest of his body.  This is why I believe there is a connection between the slide step and some arm injuries.  Coming set with most of your weight already on your back leg allows you to slide the front foot forward like before but this time with the added benefit of most of your weight behind the pitch. 
A slide step can be very effective but only if used in correct situations with proper mechanics.  Overuse it and I believe it does more harm than good.