I kinda Agree with the Op.. Considering when I did the R25 Dungeon I did it with my high Rank class ( mainly because it was my main class, and I didnt want to play as any other class). This is what I got out of doing R25 Dungeons:

1) Kills were basically 1 hit kills
2) Yielded like 10 sp per kill :/
3)It made the Dungeon Boss WAY to easy like you didn't need to Organize anything to win (which is something people were complaining about when there wasn't a Definate role for each party member).
4)What Purpose would you go there for if your a high ranked Character? Beat up Mobs and gain nothing out of it?

At the very least we should get sp from the Mobs at the R25 Dungeon, and if people want to rank up the High Ranked Characters and still do that particular Dungeon, I say They should Cap the R30 Dungeon to 30~35 Range whichever they would like to seem fit.

Thats my opinion Anyways