From what I understand, those double-deposit bonuses are targeted to whales that will have higher chance of achieving the wagering requirement. Small time rollers will not even bother to avail it.

Well ... as i know those high wager requirement is set in % of deposit. For example if you deposit 100$ you get 100$ bonus but you need to wager deposit x100 =10 000$. So it does no matter if you are whale or not. You have the same chances to profit/loss from this.
To my knowlede it brings lots of newbies to casino. Mostly those that did not try gambling before.