The way I see it, when creating a new untested control scheme, there are three important metrics: does it make it easier to use, does it add functionality, and does it make it more fun.
Nothing was made simpler or easier to do by removing buttons - in fact, it was quite the opposite. A control scheme that registers rolling and occasionally basic walking as sword slashes, and then sword slashes as rolls or basic walking, is not a good one.
It didn't increase functionality either - pretty much everything Link can do can also be done in the other Zelda games. The little added functionality in the game - the ability to draw paths for some items - could have easily been done with a d-pad or analog stick since you can't move while using it anyway.
The last one is the most subjective, but the most important; even a control scheme that makes things more complicated and removes functionality could be fine if it's still enjoyable to use. But... it's not. Having every action mapped to the touch screen is more frustrating than fun. There is no added enjoyment from tapping on an enemy to attack him compared to using a button, or from guiding Link indirectly by pointing him in a direction compared to guiding him directly with a d-pad or analog stick.
In short, it adds absolutely nothing. So what was the point? Why even bother with the full touch screen controls? Just because it's on the DS? Having it control like a normal Zelda and using the bottom screen for a map and quick access inventory may not have been as different or exciting, but it would have been a better use of the DS' features, as it would have both added functionality and simplified stuff.