So, since holding-type armlocks are not really the best of our self-defense arsenal, what is? Ukemi (the ability to fall without being hurt) is the single best self-defense there is for the simple reason that you will slip, trip, and/or fall many more times in your life than you will be attacked. In aikido and judo we teach several types of falls and we work constantly to refine these skills and make them habitual and reflexive. There are two types of falls in particular that occur over and over and deserve the majority of your practice time: the forward roll and the side fall.
The forward roll is a response to tripping - your feet stop unexpectedly and your momentum keeps your body going forward. My instructor in college has had numerous people come to him after learning forward roll and tell him, "guess what saved me the other day" type stories. Typically they are running and trip over an obstacle but have time to turn over. Often they will pop right back up to standing before they realize what happened. One time a student of mine fell out of the side of a bread truck going about 15 miles per hour, rolled over, stood up, and calmly walked back to the truck telling the astonished onlookers, "Don't try that at home. I'm a trained expert!" But I think he was amazed at that reflexive fall himself.
The other skill that will save you over and over again is the side fall. This is a typical response to slipping, as on ice or slick mud. Whenever you slip, one foot almost always gets at least a little more traction than the other, turning you somewhat as you fall, so the real trick here is to land on your side and keep your hands out from under you. This fall has saved me personally twice - once on ice and once on mud. I didn't even know what was happening till I was on the ground and realized that I'd landed properly. It has also saved my wife once when she was about 7 months pregnant with our second son. She twisted her ankle in a pothole while waddling across a parking lot. Everyone around her was shocked and horrified at the sight of the pregnant woman falling. But she landed properly and climbed back to her feet with a resounding, "I'm okaaaay."
So, since slipping and tripping are so much more common than being attacked (at least for most of us), these falling skills deserve continuous practice.