Divergent Spell Echo Support

I'm a bit confused about the value off Divergent Spell Echo...

Say we have 1000 base damage, with regular Spell Echo (10% less) will deal 2 x 900 = 1800 damage.

With 20% Divergent Spell Echo (30% less) the first cast will deal 700 damage and the (final) repeat deal 1120 damage (700+60%), which amounts to a total of 1820, correct?

However, if I apply Divergent Spell Echo on Path of Building, both average damage and total DPS are significantly lower (appr. 10% and it get's worse with increased quality).

So what's the point of Divergent Spell Echo?

There is a certain Harvest Unique that makes all repeats of a spell unable to land a critical strike, while making the last repeat always deal a critical strike. Perhaps in the future we will see more bonus that applies to the last/non-last repeat.