Yeah the main benefit of the salvage rig is that it's purpose built to buff your salvage in as few ship slots as possible, whereas shepherds can also fight. Stacking them up provides harshly diminishing returns though; 8 ships get you barely more than 3 times the bonus of one?? That's a tough trade unless you really like using them in battle. One salvage rig provides a more slot-effective bonus, IMHO.
As for the skill? Always remember that to unlock a single skill in a tree costs anywhere from 4-6 points after you count the aptitudes. Unless you're playing an industry-heavy skill spread, it's never felt worth it to dip into that tree. So few skills and most of them effectively just reduce the cost of things anyway.