I'm sure 343 is open to feedback but they're too far in development for major gameplay changes.
That's the point of this beta, isn't it? That they're having it early so that they can make major gameplay changes?
Otherwise, there seem to be few to no benefits for 343 having a beta this early.
The earlier they show it off, the less finished it will be and the more bugs it will have. If they want better first impressions at a point where few changes can be made, they'd have the beta later.