True, I do use drip lines just in case, but good point. I started with the internal ridge line when DD didn't reinforce the loops (just the corners) as I found the fabric was unduly wearing and the stitch holes were letting water in. Although my newer DD tarps have dramatically improved fixings I found having got used to an internal ridge line I was happy that 'for me' the benefits outweigh the negatives.
Although my tarp is packed in a skin for quick assembly, I can configure it however I like without disturbing the ridge line should the situation dictate; in good weather I put it on the diagonal, inclement I create a more traditional 'tent', with closed ends etc... I also found that because I rotate the tarp 90 deg (so I can pull the sides out if I wish) it only left 3 loops for an external ridge line, which would exasperate my original problem. Moreover, I found that I need to have an internal ridge line to support my under blanket properly as using the hammock suspension didn't create a good fit; the same with my bug net (although I rarely use it, I just invert the hammock). Because I don't like lots of lines to the tree (water again) I'd end up with one ridge line in and one out, with the same water wicking problem anyway. Aaarrgghh !!
It's probably my 20 years in the Military and having to make every one thing do at least ten things. Flexibility and all that... It's what drives my Mrs MAD!! An external ridge line for me only had a couple of uses. I have found an internal one to be the backbone of my whole set up, with many uses.
Hope that helps... I think it's what makes Bushcraft so interesting a subject. There's as many ways of doing things as there are personalities of people who do it. It's great to have this forum as a resource for seeing how others have overcome common problems.
Ed