Of the four judges, I put Mel B. closest to being the "dance act" specialist because of her runner-up status on Dancing with the Stars. She never had salsa in her season (in favor of Mambo) so I'm not sure how much I believe her critiques on Baila Conmigo or John and Andrew, but she was okay with D'Angelo & Amanda last season. Of course I have to believe she can understand pitchy musicians post-Spice Girls, but she also was part of a manufactured group and should be able to spot chemistry or X-factor (since she judges the UK Version).
As for Heidi, I think with her years with models and designers (either with Project Runway or Germany's Next Top Model), she should be able to understand presentation or how one should be able to fill the Radio City Music Hall stage. As questionable as Kelli Glover was vocally, Heidi should be able to say that the staging was poor. Possibly even her walking was weak.
As much as I hate Howie, I think that the eccentric acts wouldn't have a chance to show their skills without a "Howie-type" on the panel. Britain has David Walliams.
Thinking more about it, do the judge's personalities/specialties affect their choices of Wild Cards or is it producer manipulation? I put the two most recent seasons here since prior to that, there was a Wild Card show.
Howie: Red Panda*, Mike Super
Howard: Leon Etienne & Romy Low, Wendy Liebman
Mel: Tone the Chiefrocca, Bad Boys of Ballet
Heidi: Duo Resonance , Flight Crew Jump Rope
*Red Panda was a last minute replacement. I'm not sure if Howie would have been allowed to pick a Vegas cut or a family emergency withdrawal like Red Panda was.