B4U was popular enough to get a long version in 5th. I'd say that counts for something.
correction, it was originally a full version cut down? its rather odd that they added to a song, rather than just make the whole version first, isnt it?
If I recall correctly, there was a poll in Japan where people got to vote for which songs would get long versions in 5th, and B4U, and Dynamite Rave were the winners. The short versions are the original versions.
I say it depends on how loose you want the definition of "theme song" to be. Within my definition, the theme song would only include D2R and Dance Dance Revolution.
There are just too many songs that are available to be the umbrella anthem of DDR. It would be a better idea to have a specific theme song for every mix, because honestly after about 4 mixes that theme song will be getting really old fast and be chucked for a new one anyways.