My guess is that mid-rank dungeons won't have any relevant drops for a rank 50 therefor won't matter if you "steamroll" through it or not. Certainly EXP won't be flowing in if you have a team of 25's with a 50 in there. Also, choosing to take with you a high rank friend, or running through it yourself is in fact a choice. You can choose to challenge yourself, or have someone hold your hand through a dungeon that likely won't be relevant to your needs 20 ranks later.

It's pretty simple.

I do agree with your general idea though, of making more difficult content. I think part of why we haven't really truly been challenged is because of the watered down battle system, and general lack of content.