Bitterblossom is definitely not needed in UWb landstill. Considering just about every time you draw it it's going to be subpar compared to other cards you should be running in those slots.

Eternal dragon, decree, and mishra's factories are plenty of kills for UWb landstill already the deck does not need more. Besides cunning wish supplements your win conditions quite well and aids you in just about any matchup.

Runed Halo is another reason why you don't need bitterblossom considering halo compliments UWb LS much more. But the best thing about Runed halo is the enchantment is awesome just about every time you draw it.