To grossly oversimplify, DIR techniques are standardized. The people who created the system have found through their experience diving in some of the most dangerous and exhausting conditions that doing things a certain way helped them successfully survive and complete their dives; the idea is also that if everyone on the team does things the same way, it eliminates confusion in planning, preparation, and in the stress of the moment during an emergency.
Detractors say it's constrictive. Whether it is or not really depends on what you'd like to get out of diving, and how you weigh the advantages of the DIR system against what you want to do that it "doesn't allow." The following is definitely painting with broad strokes, but I've seen that safety- and teamwork-centric people tend to be drawn to rigorous systems such as DIR because of the focus on redundancy and team resources/communication, whereas the more gung-ho/individualistic types tend to see it as overbearing and stifling. There are always exceptions, and the above is probably not even the rule.
