Reinheitsgebot was implemented with only three ingredients in mind - water, barley, and hops. Yeast was yet to be discovered 500 years ago, yet we now know there are many varieties of them and how much they impact beer. So to be in compliance with the original law, you'd have to brew a lambic.
But other than "because I want to make a 'pure' beer", what reason is there to comply with Reinheitsgebot? Why does the definition of "purity" as assigned to beer by some 16th Century minor Bavarian government bureaucrat matter?
One of the three driving reasons behind the law was to ensure that grains more valuable for use in bread - mainly wheat and rye - weren't "wasted" brewing beer. So Reinheitsgebot can more accurately be viewed as a "Bread Preservation and Anti-Starvation Law" than as a "Beer Purity Law".
Additionally, Reinheitsgebot was not just about "purity" of ingredients and protecting the grains used in bread. For some reason that seems to be the only part of the law ever discussed. But there were other parts of the law: the German government setting the price of beer... and far more importantly the taxation rate of beer.
So Reinheitsgebot essentially was the government telling brewers "You can only use these particular ingredients because we want better grains to go to other uses, you can only charge this amount per beer, and this is the amount you'll be paying us to sell your beer."
Plus, Reinheitsgebot is no guarantee of quality. I was fortunate enough to spend almost nine years living in Germany. There are many great beers that comply. There are many crap beers that also comply. There are many great beers that DON'T comply...and also crap beers that don't.
Some German styles that don't comply with Reinheitsgebot, and the styles are world-renowned: Hefeweizen, Roggenbier, Gose, Dunkelweizen, and Berliner Weisse.
IMNSHABHAO (In my not so humble and borderline haughtily arrogant opinion) and not trying to denigrate the OP's intent, complying with Reinheitsgebot is more about bragging than anything else. I look at it this way: Belgian brewers have been crafting absolutely amazing beers for centuries caring not a bit about "German purity laws".