Not generally true. I'd have given up on Haskell yonks ago if that's all type inference was good for. In many cases, I just can't figure out the types of arguments without stopping completely and thinking really hard, and I usually can't be bothered to do that when I have a compiler that's happy to do it for me. Haskell gets bonus points for allowing me to insert holes arbitrarily in my code and ask the type-inferencer "what sort of thing do I need to put there?" Inference of the types of function arguments is a special case of that.

It might not be such a big-deal in Rust. I don't have enough experience with it. But if it ever gets HKTs, it wouldn't surprise me if their interaction with typeclasses becomes a pain-point without (nearly) global type inference.