The answer is Yes, I find it useful to concentrate in a single focus while taking photos on a daily basis (say for lunch, I drive downtown and spend 30-35 mins snapping away at nothing in particular, or everything). Having one lens, helps me find its sweet spots, weaknesses, strengths, and reinforce the composition.
The very first time I did this, was with a Wide Angle (19-35). My mentor, a photojournalist with over 30 years under his belt, saw all my 70-200 photos way zoomed in, and called it Zoom Syndrome. So he asked me to go around with the 19-35 and shoot exclusively with that. Do you want a bigger subject? GET CLOSER, he said.
A month later, I was shooting almost exclusively with the Wide Angle, and knew its ins and outs.
Now I carry 5 lenses with me to any assignment, but the moment I get off my car, I know exactly what the 19-35 (for example) will do, what it will not do, and whether I should even bother taking it out of the bag.
As a theoretical experiment, it is my belief that anyone with little experience, can go and buy a 1D, Canon Flash Unit, 100-400 L glass, polarizer, and when things do not work out the way they wanted, they will never be able to troubleshoot their rig.
Playing with your flash unit alone, or the TC, or a single lens, provides you with that specific experience. You become a specialist at your own lens. That is invaluable on the field, where the levee is breaking, everyone is evacuating, and five lenses stare at you saying "Pick me!, Pick me !!"... True story.