I have had Ultimate Seats on my motorcycles pretty much since Des has been in business. All of those bikes got wet from time to time and never had an issue at all with the seat foam being wet after the next day at the latest. I can appreciate people having concerns about it but for me it has been a non issue in over 16 years of actual use.
Of course it is best practice that you use the rain covers but the chance of it getting wet without one and having a big problem is quite minimal in my experience.