Using Arabic specifically in Quran and Hadeeth can never be deemed unnecessary.
First, Quran and hadeeth were said in Arabic, so when quoting them, it is best that we primarily use Arabic if we can, and then translate the Arabic. This will keep Allah’s words and His Prophet’s words intact and give Arabic speakers and nonArabic speakers who have memorized Quran a direct access to the actual words used.
Second, meaning may slightly vary from one translation to another, so we cannot say Allah said so and so when He has not said it, but the tafseer/interpretation leaned towards this meaning or that.
Third, having the original Arabic text will allow others to copy a hadeeth and paste it into their search engine of their desired hadeeth database to get the translation they are used to, instead of having to battle with different translations. With an ayah, it is much easier, since people can reference the surah and ayah number; however, people can still copy the ayah and paste it into the search slot in the Quran application.
Last, there is nothing more powerful than seeing Allah’s words quoted. This same effect is not derived from seeing a translation.