Kylotan
So really, I just want to compose 'horizontally'. [...] But I'd like to be able to drag and drop everything to work more quickly... hence this thread.

From your description, it sounds like you have all your tracks setup beforehand and they don't change much.  This way you don't have multiple instances of instruments and plugins used for identical purposes.  Within your tracks you then have your various sections/ideas (made up of groups of clips spread out vertically across those tracks) which you then slide back and forth horizontally.  You assign markers to these vertical groups because that lets you easily jump from group to group using the marker functionality, something you can't do with just the groups themselves.
 
So there're two requirements.  One is to be able to group clips across tracks to keep them together and to help in selecting and moving them all at once, and the other to be able to jump the Now Time from one group to another.  Markers, as they currently are, are only useful if you don't slide anything horizontally because they won't follow a group of clips, since there's nothing connecting a marker to a group.