Average non-crit for DS was about 1350 with a deadly grace of 1350 X 0.51 = 689, a total of 2038 damage.
So, let's say a warrior has 70% ish crit in a raid.
Dual Strike deals 2038 average non-crit damage with 70% chance to deal 3534
Rising Waterfall has 100% chance to deal 3142
I'm assuming we could find an average overall damage for Dual Strike and compare it to Rising Waterfall
So... Let's obtain the difference between 3534 and 2038 and add 70% of it to 2038.
2038 + (3534 - 2038)x0.7 = 3085
3142 > 3085
SO using Rising Waterfall when Paired Strike buff is up is better than a pure DS spam. Unless my math are rusty ! If you're higher than 70% though...I suppose it becomes bad. Also, the comparison gets bigger with lower point in the tree.