I'm going to go against the grain here and say do this deal. First off Claude Giroux is already one of the best fantasy hockey forwards. He is still really young and I wouldn't be surprised if he was the best forward in the nhl in two to three years. (Not kidding, especially considering Crosby's issues, Datsyuk getting older, and Ovechkin not being the player he was two years ago.) Rask has posted better numbers than Thomas this year and already looks just as good if not better than Thomas. I see them switching roles next year or maybe even this year if Rask keeps up this play. So basically your trading an elite goalie for a goalie for an elite forward and a goalie who will be elite in a minimum of two years, maybe even one year.
Second there is definitely a chance that Rask starts more games than Thomas next year. Why can't he? It's not like Rask is some terrible backup goalie. He's probably a starter on at least ten teams in the nhl if he wasn't with the Bruins. That's being pretty lenient too imo.
Third how is there a 0 chance this trade benefits him in the long term? Rask definitely has the potential to take over the starting job in Boston next year. Even if he doesn't you would still be getting an elite fantasy hockey forward.
The only way I wouldn't do this is if you really are going all in this year. Even then I would still have a hard time deciding. Giroux is a great forward and Rask should be getting more starts at the end of the year with the Bruins looking to rest Thomas for the playoffs.