I don't see why either. Guess no up tilts for the pro players, then.
You can tilt without tap jumping, just don't mash the stick upwards. Just push gently off center, the same way you get side tilt or down tilt without smashing.
Tap jump being optional is fairly new. utilts, usmashes, and uairs have always been around and possible.
And now some knowledge drop:
The way smash attacks work with tap jump is with a mechanic called jump canceling. During the initial frames of a character's jump animation they can cancel the jump into an up smash or up special. I haven't actually tried this in any other iteration of the game, but in smash for 3ds this is also applied on aerials; you can mash the stick as hard as you want in the air and not accidentally jump as long as the attack button is used within a certain number of frames.
The real benefit of jump canceling is that it allows you to use an up smash or up special from shield, which would not otherwise be possible. You're actually jumping out of shield but then canceling that jump into an attack. This is has been an essential technique for certain characters in the past. In the past it could also be accomplished on a cube controller by using a claw grip so you could hit a jump button and the c-stick at the same time, but since 3ds lacks a c-stick its no longer an option.
While using a jump canceled up smash or up special may not be as important as it used to be for relieving shield pressure since shields break so quickly that "pressure" may be the wrong term, being able to use an up smash or up special out of shield, frame perfect without first having to wait for the shield to drop can still be an essential counter attacking technique.
TL;DR
I'm not saying it's the only way to play. There are some players who get by at the highest levels of competition that use little or no advanced tech or movement options. What I'm saying is that tap jump adds options to the game that are not otherwise possible, and the stock response of "turn off tap jump" when people ask for advice to improve, is incorrect.