We already have that for some blocks, such as BROADCAST and ASK. But we started down that road with some hesitancy, because we think that users may find it intimidating. I'd prefer not to have huge families of blocks (everything that does a comparison, e.g., CONTAINS) be loaded with optional inputs. (You could imagine a deep/shallow option for CONTAINS, for example.)
IDENTICAL TO is for special cases if you reject the idea of worrying about speed. But if you're doing media computation, and so you're constantly tinkering with the pixels of a costume, for example, then it's a big win if you can get away with IDENTICAL TO.
Historically, Lisp solved this problem as you suggest, with optional inputs, but only after it developed keyword inputs that aren't dependent on being in a particular order. (foo bar baz &compare: eq) There could be a dozen optional inputs and you wouldn't have to know about any but the one you want. That's harder for us to invent in a visual language.