The gasket doesn't need to be very tight because the pressure of the water is going to press it against your neck.
I don't know, Dee. That may be true if your head is submerged, but if you are swimming, that pressure is going to vary and you may scoop water in through the neck gasket. (Been there.) Also, if you are in heavy surf, the firehose pressure of a breaking wave crashing down on your head can force water in through a loose neck gasket. (Been there, also.)
I've been very cold and wet in my drysuit when I had a loose-fitting neck gasket. Have since replaced it and plan to keep it quite snug, at least for the type of paddling I do.