Hello TheElamite,
We all use Cubase (Nuendo's brother, mentioned in this thread by Utora).
That being said, this is just a coincidence, and you should feel free to use whatever tools fit you the best in your professional ventures. All DAWs do pretty much the same stuff nowadays, it's more an issue of flavor than functionality at this point.
If your dream is to become a professional media composer (with Riot or else), I suggest you get acquainted with pretty much everything out there, and make sure you make wise investments (composer rigs can be bottomless money pits!). Besides your DAW of choice, you'll most likely need to use Vienna Ensemble Pro at some point, as well as engraving software such as Sibelius or Finale. Some knowledge of Pro Tools is always helpful (I keep badgering my colleagues Vonderhamz and Riot Eno with PT questions :P ).
Best of luck to you!