The problem with saying that a show has to be frozen to be reviewed limits the creative team more than it limits the reviewers. What about the case of Next to Normal at Second Stage, they were rehearsing every day throughout the run and making changes. Does that invalidate the reviews that were given?
Dreamgirls and Les Miserables are two other shows that had drastic changes made after opening.
There are many shows that are effectively frozen at the time that previews start, and many that continue to change until the day that they close.
If the Spiderman creators were smart, they would use the criticism that was given and address the problems that were identified (they may be doing it, I don't know), and then invite the critics back. It could work wonders.
Whatever happened to class?