I think the only way to rationalize a trade is to look at it from 2 different approaches:
1. The current value of the player as an asset to his team. (defined by: NHL production, Tangible stats, Chemistry with other players.. ETC)
2. The potential value of the player as a long term asset to his team. (defined by: AHL/CHL/KHL production, Potential, Age, Skillset)
In the case of Calle Jarnkrok, you identify a player who had virtually no value to the Detroit Red Wings as a current asset. As Ken Holland stated, he was never going to make this team while Datysuk, Zetterberg, Weiss, Helm, Andersson and Glendenning were playing ahead of him. And based on his physical tools, we was never going to make this team as 3rd/4th line Center either - meaning he would have had to play in a top 6 Center role (although I am unsure of his ability to play either wing) which wasn't going to happen anytime soon.
As a long term asset, he would have continued to be affected by playing in an organization with a deep pool of players in front of him (at the same position) - that fact on its own brings substance to the idea of him playing in Europe. The problem for the Red Wings was continuing to limit his value as a long term asset by keeping him in the minors at the age of 22,23,24.. etc. Collaterally, you are limiting his "trade bait" potential by doing so. So while you are banking on him being a very good top 6 player one day, it also makes sense to maximize his long term value through trade.
Although I admit the returns for Vanek, Gaborik, Luongo and Moulson seemed to actually be less than what we traded for Legwand - the trade becomes a very good deal if David Legwand decides to re-sign in Detroit. Eaves is a healthy scratch right now and was gone in July, this years (mid-late) second round pick comes in a weak draft, and Jarnkrok has yet to play a shift in the NHL. David Legwand has had what I consider to be "deflated stats" playing in Nashville's system and can provide right now what Jarnkrok scouts think he might be able to provide one day.
Time will tell, but don't write off a guy who's been to the playoffs 22 years in a row, he may know more than we do.