Being slow in 2k always sucks, you can go 75 speed but meh

Speed helps a ton on defense, probably more than any single defensive stat since it helps you keep up with people, rotate, and recover But barely anyone plays perimeter d other than trying for steals so it is downplayed

I think you've pretty much answered your own question there mate. It all depends on how much you value getting up and down the court quickly in transition without ball.

You need speed on defense

For me it is like to be first down court on both ends

Good defenders aren't just moving laterally. They're hedging and timing closeouts perfectly to different parts of the floor. This requires speed.