The one advantage that extends provides is less code bloat (if you use them right) in the compiled product - rather than inserting the same code, extends will add the select to the list with the previous selector. So if you were to write something like this:
I'm picking up what you're putting down, but I was thinking more placeholders vs. mixins. Although I guess placeholders would avoid this code bloat too, and they can be used as selectors.
.hello { color: red; }
.world { @extends .hello; background: blue }
...it would compile to something like this:
.hello, .world { color: red; }
.world { background: blue; }
na'mean?