No, they are not.
1. If the code doesn't conform to the contract, it will fail on the contract boundary, with a well-defined error. If this is useless, then `assert` is also useless, which it is, of course, not.
2. With a sufficiently well-designed language and sufficiently smart compiler, you can move some contract checks to compile-time. See Racket.
3. If your language supports both static and dynamic typing, the contracts are a dual of static types, which lets you interface the static and dynamic parts of the code seamlessly and automatically (in both directions). Again, see Racket.
Meta: I wonder, why it's mostly static-typing proponents who aggressively evangelize, insult the other side, are 110% sure they're right even though there is no scientific evidence and so on. Could it be the bondage&discipline approach of static typing just appeals to people with a certain mindset, who are statistically more probable to engage in such behaviors, no matter the subject?
1. With pencil and paper, you don't need to wait for a smart compiler - you can get started proving things about your programs today!
2. I can totally see what's coming next: “Being wrong is dual to being right, so being wrong is another possibility worth exploring”. Right?
> Meta: (slander)
No comment.
I really wish more languages took this to the logical conclusion and implemented first-class contract support. It seems work on contracts stopped with Eiffel (although I've heard that clojure spec is _kinda_ getting there).