You shouldn't buy stuff that must be included in the base game, and I say this because both the Trial of the Sword and Master Mode are things that could've made BotW to be much, much better.
The Trial of the Sword was obviously meant to be part of the main game, but got cut out by orders from Nintendo's big bosses. However, they didn't take into account that if they blocked the content by a paywall it would feel kind of underwhelming for many players in the end, as the Trial recycles a lot of the elements presented throughout the adventure. Had the Trial been part of the main game, general perception of it would have been very different, because right now it's quite mixed. Almost the same thing applies to Master Mode.
It's not like Nintendo cares too much, anyway, since they got exactly what they wanted: new buzz surrounding the game. But the thing here is that we end up paying 20 dollars more.