One point in favor of inference is that it gives you less to read, and what the compiler can infer, the reader usually can, too. On the other side, you could say you shouldn't have to infer while you read and having types in front of you reduces cognitive load.

I bet the folks building these languages with inference probably and wouldn't mind having fewer annotations (and being more consistent, in a sense) if they could. It's just that to get rid of some of these annotations you have to make deep changes to the language, like OCaml's polymorphic functions, and that may be inconsistent with their design goals or just hard.

Obviously folks won't agree on whether inference is a great thing or not. Whatever one's used to usually seems easier to read. I've worked more in dynamically typed langauges than statically, and (so) languages with inference feel more like home.