I've been practicing this philosophy for quite a while. According to Pavel Tsatsouline, repetitive and reasonably intense stimulation strengthens the nerve impulse to the muscles involved, making them stronger and more enduring. The technical term, according to Pavel, is "synaptic facilitation".
In other words, if you want to get better at doing pull ups, you do more pull ups! But don't try and do 100 in a workout (setsxreps). Instead, throughout the day, when you're near your pull up bar, crank out 15 or 20. And don't keep your pull up bar in the back corner of your yard where your never near it. Put it in the kitchen doorway or somewhere that allows you to pass by many times during the day. What ends up happening is you end up doing your 15 to 20 pull ups as many as 5-10 times a day, maybe more, depending on how often you pass by your bar. By the end of the day you've done several hundred without having developed the same fatigue that you would have by doing it in a workout. It's not to say put it somewhere that makes you do them excessively. The point is to build a habit, connecting "pathways" from brain to muscle, hence greasing the groove. You get better at it. It becomes easier. You adapt and become proficient at it.