By way of a speculative answer to your questions about why SK shoot 3 UD ...
If we can accept that integral skirmishers shooting out to 3 UD represent a skirmish screen that are deployed ahead of the main body of line infantry, then we are already accepting (conceptually at least) that although our solid base of toy soldiers occupies a single point, there is invisible area in front of the base in which activity is taking place.
In that regard, then, it is perfectly logical that detached SK can shoot 3 UD on exactly the same principle
... ie while the base depicts, say, the rallying point for the skirmishers, the actual troops represented are operating ahead of the base (exactly as we surmise happens for integrated skirmishers).
If all that seems a difficult (conceptual) preposition to accept, then another possibility is that skirmishers (whether detached or integral) are trained to take individual shots and open fire at longer ranges than formed troops. This is a reasonable assumption also in as much as the formed body will cause greater harm in mass shooting when shooting at shorter ranges and so likely reserve their fire until closer. Thus, "skirmishers" shoot a greater distance than volley fire.
All that theorising aside, for me, there are two
game aspects that skirmishers address really well in BE:
1. Dispersed formations and "skirmishers" were definitely one part of what distinguishes the "Napoleonic" period; if we remove that component, then we only serve to diminish the aesthetic of the game. For example, by simplifying the game to remove skirmishers then what is the difference between "light" infantry and "line" infantry formations ? Clearly there <was> a difference historically. And the effect of those "skirmishers" did feature regularly in historic account of all battles, and especially when "difficult" terrain is considered.
2. While I do like a "skirmisher" presence in a Napoleonic rule set, I would not want those to become the defining feature of game results. Right now, BE strikes a really good balance between the lesser damage caused by skirmishing fire as opposed to the far more decisive volley fire. To me, volley fire should principally determine the outcome of Napoleonic battles that we play at the scales represented by our toy soldiers. Napoleonic rules that push us toward melee before volley are misplaced IMHO. Likewise, in gaming terms, if artillery dominates a game outcome, then the balance is lost. Balancing melee vs artillery vs volley is the key to achieving a Napoleonic aesthetic for what is otherwise, after all, just a game. In BE detached skirmishers have a distinct contribution to make to game balance.
Lastly, to wrap up another over-long post, I particularly like how detached skirmishers in BE have to be employed with great care lest they become just a nasty little speed-bump from enemy cavalry striking at them from 10 UD away (column charge = no escape if the SK don't have supports or terrain to hide in). That's an awful LOT of table space to have to watch out for especially if those pesky cavalry are going to move later in the turn sequence than your detached skirmishers and can then set up that charge for next turn ! Nice.
