David St Hubbins
Well, you're not as confused as he is.
If we don't have room for all our rookie defensemen now, why would we trade for a veteran?
Provorov and Gudas are top 4, Ghost is top 4 offensively.
Morin, Sanheim and Myers have top 4 potential.
So why give up assets for another defenseman?
Assuming there are no restrictions on flipping said player to another team after the ED, it could be a way to add assets. For example, send Cousins for {dman} prior to the ED - a hockey trade. Protect {dman} then flip him for his true value, which would be the very definition of buying low and selling high. Assuming it would fly, it could be a way to turn a "meh" asset (e.g., Cousins) and an unused protection slot into some decent picks in 2018.