I am mixed about pulling all the air out of the drycase.
The pros are that it makes the touch screen easy to use and it ensures that there is no leak. With the air out, there is no way for the water to get in.
The con is that it keeps the speaker from working well. You are correct about the thing floating. I could test to see if my phone floats with the air still in it.
by the way they sent me a discount code for future orders. The card says use "keepdry" in the check out for 10% off.