arab/wb crosses are actually registerable as Arabian Warmbloods in Aus :) at the moment it's considered a "derivative" registration rather than a breed but in years to come that will likely change.
And the Trakehner studbook allows crosses to TB's and Arabians (and Anglo's) to make "purebred" Trakehners, for the purpose of refining the breed. In theory you could have a horse that was 7/8 Arabian, 1/8 Trake, but if its parents on both sides were studbook registered/approved it would be registered as a purebred.
I actually REALLY like the cross, and have met several that are simply lovely people. BIG personalities, but talented, athletic, and stunning to look at. You hope for the best of both worlds with the beautiful Arabian head and refinement, and the WB movement, power and athleticism. It's a really popular cross atm in Aus and proving quite successful in the eventing world.