It makes life a tonne easier when/if you progress into diving where you need more gas - whether technical or just a 30m nitrox rec dive.
As for the setup purely as a single-tank solution, it makes sense to have a gassharing procedure where you donate from the mouth. The 7ft hose is well practical in open water as well, as a short hose may well be a bit too, well, short.
If you're interested in diving a DIR configuration and learning about why it's configured the way it is, while boosting your skills, I can recommend the Extreme Scuba Makeover or Essentials of Recreational Diving programmes.