Units that aren't being used that much, but don't break anything aren't as high of a priority for patches. However, they are high priority for the next game. This list includes not just the Nydus Worm, but also units such as BCs, Carriers, Corruptors, etc.
Like we mentioned in another post, one of the most core philosophies for our SC2 design team is that if the change isn't completely awesome, we don't change it. Reason being we don't want to take away from players who are actually using the current versions of the units in a fun way of their own. And we're in such early stages that it's really difficult to say that we have a good solution for any of these things, but we are trying our best not only to work on supporting HotS, but also thinking farther down the line.
http://www.reddit.com/r/starcraft/comments/208le3/starcraft_ii_heart_of_the_swarm_balance_reddit_qa/
스2게에서도 저번에 이런 이야기 본 적있는데 확실히 한번 더 말하네요