Articles by Ron Johnson, Sheila Taormina, and Terry Laughlin present some compelling arguments on training long, training fast, or training for efficiency. My summaries may be a little over-simplistic, but I hope to have captured the essence of each author's ideas.
Ron Johnson (former coach at ASU and Sun Devils Masters) addresses the argument of doing high quality or "under-load" work (making fast swimming in a workout easier) or high quantity or overload work (making swim training harder). Some coaches see them as mutually exclusive. You either train hard or train fast. However, more and more are finding the value of mixing the two. The signs are in the continued fast swimming over a series of competitions, as in recent World Cup meets. These ideas are probably more applicable to events of 200-meters and below, events where speed is a bigger factor than endurance, but even in the 1500-meter races, speed is important. The right mix and timing of performing overload and under-load work is a potential key to faster swimming.
Terry Laughlin (from Total Immersion) presents his point of view - that it is vital to swim as efficiently as possible by practicing three key concepts: power from the core, hands lock in place in the water, and gain speed by using the body, not the limbs. Depending upon the swimmer's goals, many will get more out of being more efficient than they will from trying to get a little faster by expending a lot of energy. Decrease stroke count and get the most from every stroke. Be certain get as much out of every stroke as possible. In the long run, it's better to be technically efficient at swimming.
Sheila Taormina (She has made US Olympic teams in swimming, triathlon, and modern pentathlon) presents her views on coaching swimmers to swim fast by locking on to the water and using the athlete's muscles to get the body moving - and to keep moving. No gliding! Not a new concept, but to swim fast once sound technique is in place, more is needed than to simply keep making the technique better. The emphasis needs to be on developing the physiological systems involved - develop fitness - and use that fitness to increase power output. Find the right balance between stroke rhythm and stroke count without any pause or glide. Keep something pushing on the water (or holding the water) at all times. Practicing good technique and increasing the ability to exert force on the water results in faster swimming.
You have to make decisions when you train. Why are you swimming? What do you want get out of the work you put in? I think a blend of all of these ideas can be used. Start by developing good technique and building a solid endurance base. Then keep your good technique and begin adding speed elements to your training. No one would argue against the idea that the most efficient, most powerful swimmer is likely the best - but how to get there?
Deciding exactly how to train - what you think is right - is one of the fun things about swimming. Who's to say there is only one way to do things?