Bottom row

Why does collada export the bottom row for matrix animations? Is there any case where the bottom row would not be 0 0 0 1?
Just wondering

Can you give an example of what you mean (I assume you mean a <source> inside an <animation>)?

COLLADA <node> coordinate systems can be projective. In practice you might not encounter a lot of animated content driving projective transformations but it is possible, i.e. camera animations.