This is exactly right. And a good example is special purpose Turkey shotguns. they rarely have longer than 20" barrels and yet they shoot the tightest patterns of any other shotgun.depending on your intended purposes (bird hunting, sporting clays, skeet, trap, etc.) an 18 inch barrel compared to a 26 or 28 inch barrel isnt going to make a whole lot of difference. if you're claiming it as a "sporting rifle" on your paperwork and are actually going to use it in your home then i can see a possible complaint about the extra 10 inches but otherwise it isnt going to make much difference. despite what hollywood says, shortening the barrel isnt going to expand your shot spread dramatically...not movie dramatic anyways. the spread can be more effected by your choke. here is an example chart that shows the differences.
http://media.photobucket.com/image/recent/Rastoff/ammo_pattern_spread.gif
And in a situation like that are the authorities really gonna care if you take a hacksaw to your shotgun??Alas, lasers are also illegal.
Actually my weapon collection is built up from a survivalist perspective, not strictly home defense. In the latter case, you'd indeed be better off choosing a corner and sticking to it. However, the scenario I'm envisioning is a total collapse of society, due to shortages of basic supplies (food, drinkable water) or natural disasters. In this case, staying on the move and living a nomadic life would require you to be able to move around while armed.