The official C&C Facebook Group updated with this message last night.
QUOTE
The defensive class for Command & Conquer 4 will allow you to base build, so for anyone worried, we have a class for you! This is a new approach to C&C, one that is needed and we're looking forward to your feedback.
I noticed this to be a major concern, especially from this community, when EA announced C&C4 would feature "mobile bases" but it has somewhat been cleared up by EA. Still not too clear on how this will play into the campaign, but they are not taking the feature out of the game.
10 days until the trailer!