Hi Mike, away for a couple of days without the rule book so what I am about to say is totally unsupported (as per normal).
Is the comment in 5.2 made in the context of moving other than straight ahead? In which case it is there to stop people moving forward 6" and then pivoting around so far as to effectively looking back the way they have come i.e. an 180 degree wheel rather than a counter march to about face.
The about face takes half a move, you still have half a move to do what you want.
I think I would have to see a situation where you would be avoiding a charge by not allowing the centre of the enemy unit get to you (but sounds like a good tactic
), but I presume you have seen this.
I think there are some issues where the alignment of units can stop people getting in. I know of one situation at Derby (Worlds end!) where there were units on both sides abutting other units but unable to get in as they were too wide for the centre to get in. When in all likelihood they would just advance into contact. I know the counter argument would be they have to maneuver etc, but you may not want to drop from a 4 base wide unit to a one base wide unit to contact a base of an opposing unit that is not fully engaged and you may also not want to move on to a flank for fear of exposing your own flank or from falling foul of the 1" rule.
Sometimes these anomalies will happen, it is just asking how often they are and does it spoil the game. We need to be careful about getting what we wish for.