I checked Matt Elias' Article "Legacy cookbook Spring 2011" where many archetypes and current builds are listed.
2 decks run peacekeeper:
http://www.starcitygames.com/magic/l...ring_2011.html
1st is dreadstill. here the comment:
"At one point, Dreadstill was one of the key players in the Legacy format, but it hasn't had much buzz of late. However, there are a lot of reasons to consider Dreadstill today. Rich Shay brought Peacekeeper into today's Legacy format, a card that can lock out some strategies, especially Merfolk, a strategy many CB/Top decks struggle against."
If I take a look at the sideboard, I would rather worry about 4 Plague + 2 Perish.
The 2nd archetype listed there with a peacekeeper was doomsday combo.
In the SCG Open I also found an U/W landstill build with 3 peacekeeper in the SB and 1 CB/Thopter with 1 Peacekeeper (Both in Top 16).
So, I was wrong.. the card is more popular has more value than I thought...
Still not sure if it is worth to sacrifice a SB spot for it... in all lists and SB that I saw, there were many other things to worry about, which would be more likely sided in, being played more often...