G'day Jagiello
I see your point - yes, a LH element could move from directly in front of an enemy element all the way around the flank of an enemy element and stop 1 pace from the enemy. But there's an invisible force field stopping it from moving that last pace into contact, at least until the element's next bound.
In this case, I'd suggest it's a game mechanism to give the enemy one chance to react to the threat. After all, remember that DBMM is a turn based game, where *you* get to do all your movement and combat, then *I* get to do it, and so on. However, in reality, both armies would be acting and reacting simultaneously - for example, moving up a reserve to cover the flank of the element in combat. So I think this sort of rule is designed to stop either side from being allowed to do too much in one go without giving the other side a chance to do something in response. Of course, if I don't get the PIPs to move up an element to cover the flank of my element in combat, then I just have to pucker up and watch your LH element hit me in the flank (or hope *you* don't get the PIPs to do it).
Another point to consider is that this is a rule carried over from DBM (and DBA too I think, possibly even 7th Edition), so it's a well established concept. Okay, "this is the way we've always done it" isn't necessarily a great defence, but I'd be happy to stand by what I said in the paragraph above.