To me, a "definitive edition" would be one that can combine the best features of the game that exists on other platforms.
The Wii U and Vita versions have levels that utilize touchscreen and/or tilting controls. The other versions replaced these with levels that play using traditional "gamepad" controls. Some people really like the touch and tilt stuff, other people despise it. As I mentioned earlier in this thread, I'm kind of in the middle on this issue--I enjoyed the touchscreen sequences, but I wasn't fond of the levels that depended on tilting the system.
If the Switch version gave the choice of using or not using touch and tilt controls, then you could have a single version of the game that could satisfy both camps.
Another reason why the game should exist is just simply that it's a great game, and there may be people out there who haven't played the game yet and would want to do so on their new system. Or perhaps they have played the game, and want to play through it again with whatever changes and additions that Ubisoft is throwing in.