I removed them because I don't feel they are absolutely needed and also it was interfering with the exhaust I was making.
Yes they do serve a purpose to stiffen more the front attachment.
But the front sub-frame attachment already has another brace. (( 2 actually if you include the W brace that act similar to the V ))
The only big difference between the 350z and 370z is the long V brace that goes from the front main of the sub-frame attachment to the rear middle attachment.
EDIT;
It doesn't show on the picture, but the 350z also has a brace that connect both rear attachment together and bolted to the frame.
Similar to the 370z brace that is shown.
But I would keep that W brace for sure !