Interesting I should come across this thread, because I wondered a little about this myself. But here's my take on it, for what it's worth.
A few technique drills before jumping into playing can be likened to an athlete warming up before an event. Stretching muscles and getting mentally prepared can make for a far more effective time of playing with less strain. That's how I approach it. I've been using the Hanon exercises, and I find that since they are repetitive, I can take a few moments to concentrate on other musical things while my fingers are drilling away (i.e. is my rhythm steady and even? am I relaxed or tensed up? am I varying dynamics in the phrases or just plunking away like a machine? etc....)