Hi Comet. Right well I should admit at this stage that the only plugin that I've encountered and used for pixel perfect sprite collisions is eXtends so my comments will mostly relate to my experiences with that. However there are a number of advantages of using line collision over pixel perfect collision in general.
Firstly, and this should apply to any pixel perfect plugin, line collision is much much faster whilst collisions are actually occurring. This is because it only has to check for a few line intersections rather than for every pixel in the overlapping areas of the two sprites. Thus, you should be able to achieve a higher fps.
Another advantage of the LineMap plugin is that it can be used with any sprite system and any type of sprite. Some pixel perfect algorithms prefer particular image types as they have to analyse them to check for collisions (eXtends prefers bmps for example). Further, at least in the case of eXtends again, the plugin requires the use of DBP's native sprites. Neither of these is the case with LineMaps as they are entirely separate from the sprites. This is important for a couple of reasons. Firstly, to keep size down, many people prefer to use pngs in their games. The LineMap plugin will support these as well as any other image types you can load into a sprite. Further, if you're using the LineMap collision system for performance reasons, you may well also want to use an alternative sprite system such as Advanced Sprites. As the LineMap plugin is tied to no individual sprite system, this is possible.
Moreover, the LineMap plugin supports almost all transformations supported by DBP's native sprite system (and animated sprites to boot). This is an advantage because pixel perfect algorithms don't always support all these transformations.
Finally, pixel perfect relies on alpha channels which can sometimes be a bit fiddly when you are defining them yourself (as with bmps). LineMaps avoid this.
Hope this clarifies why I think they're useful. If you have any other questions of queries, please do ask.