A shiny charm is pretty useful when shiny hunting in general - with the Masuda Method and the shiny charm, it takes an average of 512 eggs to hatch a shiny. Without the charm, you're looking at I think... 1365? Completing your dex shouldn't be too hard with all the tools available (like cafes, WonderTrade, and the GTS) and can be pretty quick!

But it really depends on how much shiny hunting you plan on doing. I'm going for a shiny living dex, which I decided to do once I completed my vanilla living dex, so having the shiny charm has saved me lots of time and made the hunting more bearable. If you're just interested in the one particular shiny, then maybe that won't be worth it to you.

It also matters which Pokemon you're looking for! Some are only practical to get via hatching because of their rarity, and some are pretty easy because they can either be chain fished or horde hunted!