The best, most compelling reason for using a DSS is two words: Opportunity Cost.
Every time you're within "surface scanning" range of an object you have an opportunity (to increase your Explorer Rank)...how much you make of that opportunity is up to you. No one is selling exploration data trying to get rich, so throw money out the window right there, you're basically doing it for the Exploration Rank, right? (If you ARE trying to get rich, you could do worse than "investing" time in gaining exploration rank, because you can get some really tasty passenger missions at high Exploration Rank) In order to make the most rank for your time spent scanning you NEED to use a DSS. I generally don't even bother with any surface scans at all unless I have one. That said, since I started playing again I've tripled my total number of Level 2 scans, but that was accidental, really...
In short, if you're an Explorer you already want one, right?
If you're not an Explorer but you're doing surface scanning anyway, you NEED one even more. (Because using one will hasten the point where you NEVER HAVE TO DO IT AGAIN, savvy?)
If you're not an Explorer and you're not surface scanning why are you still reading this?
