Difficult on understanding Laziness

I found relative topic, Spec section 5.1: Ref-impl bug and/or unclear spec?

But I couldn’t find acceptable reason for using Laziness rule. I mean, I feel this rule is strange, not clear, unintelligible… but only for laziness of writer? omg…

That laziness, anyway, only works for a paragraph, that mean, on the other blocks, it always needs a front block quote marker.
Too much complexity for little convenience, isn’t it?
And this mean that writer’s experiences of Block Quote are different between paragraph and other block types. I think unified experience is very important for editing.

So, I totally disagree with this rule, but really I want to know about other perspectives.