To be honest, having a time is a necessary evil. On one side you are waiting and waiting for a game to launch. On another side you could just go about your day and come back later or around the end time given to see if the game was launch. Many MMO companies do this to make sure their consumers aren't completely out of the blue as to when the game will be out of maintenance.
Yup, I'd agree there. I think in this case though, with the time constantly moving, it's clear they may not be 100% certain themselves of when they will be ready. In that case, I'd err on the side of caution and give a date. I'd post a message that the game will be available by 8/23 or something. Using that logic, if they had posted "by the end of the day" or similar, they wouldn't have had to move the date and they would have been accurate.
I guess my main point is: if you aren't confident of a specific time, don't specify one. Give yourself wiggle room. Bugs can be tricky, and even when those are worked out there's a lot of testing before you roll out (and of course tons of users slamming the servers at once when the game is finally online will add a whole new level of complications). I think they just need to be better at managing expectations.