For those who wandered here from elsewhere, allow me to explain what banlines are. They're an illusory form of security in Second Life. A designated perimeter can be set to block specific people, everyone, everyone except friends, everyone except members of a group, everyone except those on a guest list, permit only those who pay a fee, and so on. When this is set, a kind of forcefield is not only set along that perimeter but above as well. Usually 88 meters up from the ground.
1) Someone is harassing the landowner and said landowner either doesn't not know how to block the offending avatar or there have been alts used to get past a specified ban, and
2) Residents are having sillysex and don't want anyone barging in on them.
This doesn't prevent anyone from looking in though. As my friend Peter once said, banlines simply "keep people out" with regard to an avatar's physical access.
Aesthetically, they can be an issue. They appear from within 1 or 2 meters away, which means they are not obvious from a distance so they are not an eyesore to most, but having a banliner for a neighbor can be a problem as the alpha stripe wall appears along your property and through other alpha textures such as your windows, trees, and fencing. Recent viewers have allowed homeowners to hide banlines so that paranoid neighbors are no longer a factor when selecting a place to settle. It also helps to have a property large enough that you're seldom at a border.
This leaves vehicles. When an avatar walks or flies into banlines they bounce harmlessly off. Riding into them means that ride is over.