In making these comparisons you need to remember that starting in MRU requires you to add Africa as a continent which you may not want to do given the dearth of flights there. So it may be more appropriate to compare a DONE3 ex-NRT with a DONE4 ex-MRU or a DONE4 ex-NRT with a DONE5 ex-MRU. When you do that, according to my figures, the NRT prices are less than $300 more than the ex-MRU prices- so with the MRU service charge the prices there are actually higher. Given the other hassles, it doesn't seem worth it for a DONE ticket.