Fast-twitch muscle fibers seem to get all the attention from those looking to build fast muscle. This is understandable to an extent. After all, fast-twitch muscle fibers produce greater force and have a higher potential for growth than slow-twitch fibers. But there is more to building muscle then simply stimulating the fast-twitch fibers.
If your goal is really to build fast muscle, then it is necessary to stimulate all types of muscle fibers to grow. After all, just because fast-twitch muscle fibers have the highest growth potential doesnt mean that we dont want the slow-twitch fibers to grow as well. By focusing on only the fast-twitch fibers you are cutting your gains short.
In addition, the type of exercise that is typical of any bodybuilding routine relies heavily on the slow-twitch fibers. Most fast-twitch fibers dont have the endurance for more than 10-20 seconds of work. However, it is not uncommon for sets of 8-15 reps to last 40-60 seconds or longer. This is where slow-twitch muscle fibers come in. Without strong slow-twitch fibers these sets would not be possible.
Additionally, you are stronger if you can use all of your muscle fibers together. If you are only using your fast-twitch fibers then you are not producing as much force as you could with the slow-twitch and fast-twitch fibers combined. As you should know, there is a strong correlation between muscle strength and size, so increasing your strength by using all of your available muscle fibers is a must.
Please note that I am not suggesting that anyone has to ability to only recruit their fast-twitch fibers during a set, but from some of the discussions I have heard this does seem to be a goal for some people. I am here to tell you that not only is this not possible, but its also not desirable if your goal is to build muscle fast.