What have we got to lose?
Cap space, evaluation opportunities for players to get called up and fill roles, a potential roster spot next year if we hang onto him.
As several people have said, it is really dependent on what happens with him.
So do you think this is a bad trade?
Why does it need to be established right away whether a trade is bad or good, or if it was 'won' or 'lost'?
Several people have qualified their opinions regarding the trade, stating that if he is a stop-gap solution due to all the injuries the Canucks have incurred, it's an okay deal. If the result is that Spooner ends up eating a roster spot to finish this year and going into the next, it is a bad deal.
EDIT: And I should clarify that the latter being bad really depends on your opinion on the team and what you feel is needed for the future. Again, based on past track record from this management crew, I'm not keen on another spare parts type player who isn't particularly standout anywhere getting a chance with this crew. We already have one of those types of players and his name is Granlund.
If you feel that Spooner is a good player, please explain why you're excited in him potentially taking a top 6 (top 9?) spot. What did he do in Boston that makes you think he can replicate his success here? Is he going to be in a similar position to succeed here in Vancouver?
Since you are wrong that we aren't all clairvoyant, we have to contextualize things and not just operate on a binary "trade good/trade bad" system.