Here's an old plane of sky clear over the course of 135 mobs.
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%: 41 pierce, 34 backstab, 25 slash.
It's helpful to think of rogues as warriors with a nice kick button. Warriors actually do very good damage. It'd just that Backstab is a moderate sum of extra damage piled on to what a warrior could do. Main / offhand ratios are where most of our damage comes from, by far.