Intended Security Hole?

[Manfred Nowak]

>From the docs, i.e. function.html:

> Functions without bodies [...] enable implementation to be
> completely hidden from the user of it.

This means also, that the implementation can be changed during runtime 
by "overriding" with another implementation, which might be malicious.

Why is it a good thing to rely on the reputability of an unknwon coder?

-manfred