If an effect instructs you to "double" a creature's power, that creature gets +X/+0, where X is its power as that effect begins to apply
Millicent, however, costs less for each Spirit you control, and provides you with additional Spirits to either snowball out of control or have insurance against board wipes
If you wanted to summon Nicol Bolas, God-Pharaoh as early as turn 1, this scheme could, potentially, make that possible
While the post calls this new drop weird and ridiculous, its hard to view it as such nowadays
1 Deep Clue Sea Morska, Undersea Sleuth and Sophia, Dogged Detective Easily the best deck among the four to be released in Murders at Karlov Manor is the Deep Clue Sea Commander deck