Hey Nick,
Code completion helps everyone! Even us web designers/developers. Only the developers can say really how much it would bloat the application. I say if it's not a feature of the app make it a plug-in so the "hard-core" programmers who don't need/want the code completion for web languages/technologies don't have to have it slow them down and the rest of us Designers/Devs can take advantage of a feature we've come to know and love.
With regard to cssEdit, I don't like having 15 apps open at once. The fewer the apps the better for me. skEdit, in my opinion, does better than cssEdit (again because of code completion) and makes my css dev time so much shorter. I think TM needs to be flexible and fit different types of work by using some sort of plug-in model to add the extras we need and want.
-- jheyer