"When declaring a barrel roll or boost, Dash Rendar must choose whether or not to use his ability before measuring if he would overlap an obstacle."

I'm trying (and failing) to figure why this is a thing. Why would you ever not use dash's ability?

Because if it's your LAST boost or BR in the round, you might not want it to be successful if it lands you up on an asteroid and makes you unable to shoot.

Because if it's your LAST boost or BR in the round, you might not want it to be successful if it lands you up on an asteroid and makes you unable to shoot.

This.

You might think it will fit, but you could put yourself on another rock. Dash allows you to end up in this state where other pilots do not. Pretty much, "no takes-backsies".

Dash is my single favorite pilot in the game. (Mechanically I enjoy his ability and play style. I was also an obsessive Shadows of the Empire player on N64 as a kid.)

The fact that his ability is a may doesn't come up too often but can be fantastically useful in corner cases like the ones mentioned above. There are times where you can possibly get into a better position by boosting but are risking clipping a rock and leaving yourself with no attacks. At these times you loudly declare "I am not using Dash's ability!" Then you can comfortable attempt your Boost without fear of a red diceless round sitting on a rock like a goon.

You can even use it when checking Barrel Rolls around debris or rocks to get an idea of where exactly a Barrel Roll will place you. In general I don't have trouble judging Barrel Rolls but it's another consideration.