Of course the PIC is legally responsible for the safe conduct of the flight but in order for him/her to conduct that flight safely he/she needs to be provided with all the information relevant to the flight by those who actually have the information! It also needs to be provided as soon as it becomes available either from ATC or the airlines Ops Centre or whatever it is called. That doesn't remove the responsibility from the PIC for the ultimate outcome of the flight but it makes the decision making process a whole lot easier when the G part of GRADE is a complete picture in real time of what is happening with the weather. If the weather that has been provided states that there are no requirements particularly for the alternate then its not too much to ask for the crew to be updated when that is no longer the case, or are the crew expected to ask every five minutes for the latest update?