I'm embarrassed to admit it, especially amongst all of you CAD gurus, but...
I know little about what PLines are REALLY for, other than I can make a 'thick' line with them.
craigr
I'm like DGCS, I don't draw lines. I have a hotkey that triggers the 'pline' command assigned to one of the extra buttons on my mouse, and that's all I use to draw with.
The benefits for me are many.
1) When hatching, I automatically have a closed boundary with no chance of mistakes that leave gaps. It is a closed boundary I can select, rather than clicking a point and hoping Autocad doesn't crash trying to calculate the imaginary boundary. This also leaves the hatch associative 99% of the time, and allows the hatch to stretch along with any stretches I apply to that polyline, saving repetitive processes.
2) There are times when we override the thickness for similar reasons.
3a) When working in 3D, a closed polyline will result in a solid when extruding, whereas a simple loop of lines will result in faces, last time I checked. Not desirable for me.
3b) When a solid is created by extruding a closed polyline, you can edit the profile of the extruded solid by using the grips from the original polyline.
4) Don't have to worry about forgetting to select a segment, like you said, when moving. If you have a polyline that goes through a hundred other objects that makes it impossible to use a left-to-right or right-to-left selection window... you don't have to worry about going through and picking each line segment... it's all one big pline.
5) Area, perimeter, and other geometry calculations are in the properties window already, or available with a quick tap of the 'LIst' command.
6) eh there's probably quite a lot more that have become so second nature that I can't think of them at the moment, but this is all that comes off the top of my head.