La Traviata and La Boheme were D&W's SD choices, not their LP choices. And they were chosen because the Golden Waltz is a Viennese Waltz, and they wanted to go with a period piece feel to their SD, especially after USFS told them at Champs Camp to ditch their original Amelie SD. And with good reason. (Very noticeable in the scores of Monko & Khaliavin, who won Jr. Worlds, but did not do the VW skills on the correct beat of the music. Two judges slammed them for it in the Timing/Interp component.) P&B also ditched Amelie and used Lara's Theme instead in midseason. C&P did not ditch Fallin', for example, and IMO, their SD scores at 4CC's and at Worlds might well have been better if they had. And one judge scored I&K siginificantly lower for Timing/Interp than the other PCS grades, probably for similar issues.
It's interesting that you mention K&P, because they were the originators of the GW, and if D&W's music caused you to think it would have been a good choice for K&P, then their choreographer made a good choice.
As to the LP, 3 of the top S/Z teams were using ballroom this year, at least in part because there had been a great deal of discussion/agitation last year about wanting happy, upbeat non-Requiem-For-a-Dream sort of pieces with a ballroom feel. That rule did not pass, but that it was proposed showed a strong faction looking for a more traditional feel to music. The Shibs "Smile" was both ballroom & happy-happy! V&M did Latin (samba/rumba), and D&W did tango for the LP.
Now as to why D&W chose tango, they were very clear about it. They wanted to work on several things that they thought tango was a good vehicle for them to learn from:
1. They & for that matter V&M as well, although not as severely, had been criticized for not having enough transitions in their FDs. It was said that they could not skate with such speed if they had had more transitions. This was said to affect their Skating Skills and Linking Movements PCS component scores. In consequence, this year's tango FD was packed with transitional movements, and still skated at high speed, by comparison with last year's Phantom FD.
D&W WC 2011 Skating Skills 9.57 Linking Movements 9.54
V&M WC 2011 Skating Skillls 9.32 Linking Movements 9.04
by contrast from 2010
D&W WC 2010 Skating Skills 9.50 Linking Movements 9.35
V&M WC 2010 Skating Skills 9.60 Linking Movements 9.50
2. They wanted to do an FD that was not dependent on a big story, like Phantom or Samson & Delilah, or for that Matter Eleanor Rigby & Polovetsian Dances, to show some versatility, and also to force themselves to work harder at doing an FD without a particular story line, where they would have to tell a story about a relationship. And they stepped up the mime & acting lessons to help them in this endeavor. And they made progress on this all year, although they still have work to do, I think.
