Is it good practice to force students to keep their test booklets open for the duration of an administered test? I think yes.
Keeping the book open serves multiple purposes. Students who are thorough in the execution of their work benefit from such a strategy for they are no longer under any sort of peer pressure to close their test booklet or finish when the majority of the class finishes.
The student who is finished early has the opportunity to go back and check over their work, something that isn't often completed to the fullest extent when the book is allowed to be closed at any time.
When a writing draft is staring you in the face for 30 minutes, you're more apt to look it over and make some changes, perhaps even write a bit more supporting the topic. I saw this happen with 10 minutes left to the test.
I've seen this strategy benefit students in our test taking session today and I'm glad we all agreed to try and implement such a strategy. The kids can always put their head down and take a nap if it is really bothering them.
Of course, the down side of booklets being left open is it requires active proctoring. Moving around to be sure students aren't moving ahead or looking back to previous tests is very important and requires a bit more effort. Guess I'm willing to make that effort - I should probably have been doing it anyway.
I also feel I haven't had disruptions due to the fact students are not allowed to read a personal book. I guess I'm thankful I'm teaching a group of really nice, well-mannered kids.