It depends on the formation. I tried moving two groups with a space between them using alt/right click, and the AI reformed my two groups into a single line group. Also, the AI doesn't recognize an echelon formation as organized. It will change it every time.
My groups have NEVER been changed, that is unless I make a mistake like forgetting the Alt key.
You loose your spacing when you do that, and with the speed of cav your archers are vulnerable until you re-establish the spacing. Also, if you see enemy cav coming in and try to intercept it, your cav doesn't move for 2 seconds. That makes the spacing even more critical.
My army stops, archers are grouped (approx 1 second) I set their destination to fire and as they begin to march, the rest of my army is moved to support (approx another second) I would not consider my position vulnerable at all. Plus, there are different factors present now so the common Pavs out in front with a big gap between them and your army is not necessarily the best deployment; so my archers may even be in a group with spears, which are able to protect in more ways than one. As for my cav not responding for 2 seconds, I have stated before that I have never seen a 2 second delay in reaction to orders. When cav does delay is when a moving unit is in front of it, the cav waits for a clear path.

One thing is certain, MTW II will not be using the MTW engine

...........Orda