Or rather, why force folks to replay 20->30? Why not simply let them spend 6M karma to buy the feats directly? The later would provide a very strong enticement for people to play at 30. It would give them a chance to get a lot of use out of the L29 gear. It might even give folks a reason to stay long enough to acquire and use raid gear.
I'm just guessing, but I suspect dev-reasoning was something on the order of "back to 20 gives life to older content and gives new epic characters more to group with." That fails a little bit with the lack of XP penalities for Epic groups -- only helps for the low-epic reaper groups. But, reaper isn't newbie friendly.
Does it slow down karma acquisition? Probably. But if there was a 3-day lockout on a karma-buy, the maximum rate would be identical. So. Hmm.
What do you think we gain from rerunning 20->30 over a direct buy with karma without a rerun? Where "we" covers just about any subdivision of DDO you can imagine: we as players, the game itself, SSG, fresh newbie epic chars, die-hard RXP'ers, etc..