You've seen the benefits of outlining for one class, you haven't gotten benefits for the others. That's really all that should matter-do what works for you. That said, I have found outlining for every class to be tremendously beneficial.
One thing I found about contracts in particular was that outlining from the beginning to end of the course really helped me think about the contractual relationship chronologically--from the promise to consideration to breach to damages. For the more tactical civil procedure courses (the pleading-->judgment/appeal type), I also found it helpful. For courses that seem more scattered, I've found outlining slightly less helpful, although for me it's worth the time.
The two obvious negatives with supplements and upperclass outlines are suspect applicability and suspect quality. With the outlines, unless your teacher wrote the casebook or never deviates from it in class, and you have a thorough supplement that tracks the casebook, they will be off in certain points. With upperclass outlines, make sure they're recent and you know the source. Anyone can hand you an outline; not everybody actually does well in the class. Often outlines are 2nd- and 3rd-hand, and if your professor covers a lot of new material, might be defective or misleading.
I found knowledge gained from other people's outlines and from supplements to be overly simplified, and less likely to stick in my brain. But obviously, if it works for you, don't let me or anybody else convince you that you're studying the wrong way.