I'm one of those that keeps the minimap constantly open, because it gives me a quick overview over the area around me.
And it also works as a compass to get directions and bearings sorted out easily.
I love to sail in SL and spend time on the seas and channels, and the minimap gives me feedback on crowded regions (which can be laggy or otherwise troublesome, especially if you sail in a group with others) and a sense for land/water and objects covering these (something grey in a water channel is never a good sign
). The latest addition of the physical objects also makes it easy to identify lost boats and alike. Really valuable on today's seas!
It is true most of the information can be found in the usual map and the radar, but the minimap itself is silent and gives a visual feedback that is a good summary of what is around.
What would be good (for me, and other sailors/aviators as well of course) with parcel borders is that it would help identify paths through regions, like you also mention (and yes ultimately it would be fun if it marked out the protected path, but that is never going to happen in a 100% certain way the way SL is constructed, we have to rely on some human brain processing there).
It is true it would never show the real status of the parcel in question (and I would certainly not trust the parcel data for an overlay anyway, too slow indeed) but even in a small size the outline on the minimap would be helpful as a pointer, and it is frequently mentioned in different groups that if the viewer have these then they are better turned on as a help.
I'm not saying it is a must or even a request, but perhaps if it isn't a too big chunk of bloating coding you could give it another thought at some convenient time.
Ultimately your decision, and I understand your hesitation about it. But the map is resizeable, has three zoom levels and it could add some usefulness to it for us on the move.
And if I was to implement it I would sure make it a toggle like the other features in the minimap so it can be turned off to unclutter (possibly default for that matter, come to think about uncluttering/cluttering).
OK my L$0.02 of blabbering and thoughts.
And to recap, I'm a very happy user of your viewer. Especially with the latest additions ^^