I think it's really an individual preference. There's some benefit to making an outline: you're forced to organize the material, and in doing so you're studying the material in a way that goes beyond what you'd be doing if you were simply looking over a commercial outline. In other words, when making your own outline for contracts, and deciding which cases to place in your offer/acceptance section, you're looking the content for that aspect of the course.
I think a problem might be when someone takes so much time constructing the outline, and constructing the outline becomes the work, rather than learning the content. By buying a commercial outline, assuming it's a good one, you've basically paid for someone to do the work of constructing the outline for you, and you can concentrate on learning the material within the outline. So, there is definitely a benefit to that.
Personally, I'm in favor of buying the outlines, but I do appreciate the benefits that some people might get in making their own.