At some point, you'll end up with more MA and LK than you need for PvM, so R/MR will eventually OHKO despite how weak it is. Same with storm. If you get RMR (booster), it'll just make it OHKO earlier, and who can say no to a 0.3s OHKO spell?
The splash will also eventually OHKO if any secondary targets are close enough to the primary target. Since getting RMR doesn't add to the casting cost and does not detract from MR's accuracy (as long as RMR's level is not less than MR's level), there's no penalty for improving your chances at a quick, multiple-monster OHKO.
The dark type probably needs MR least of all since his/her spells will OHKO anyway, but, if you can 2HKO between MR and storm and if you have the points to get R/MR, why not?
Temp inactive again...
GigaBowser, 4411 HP melee dark/fire/lightning wizard, 193/204
Deudalephon, 4411 3:1::LK:WT melee water/lightning soul master, 150/165
Hemathion, 4411 3:5:4::WT:HP:MA melee dark lord, 140/140
Anchors, 1414 MA priest, 244/250
Vadise, 1414 MA wind/earth soul master, 210/216
IreneAdler, 1414 MA fire/lightning soul master, 154/152
So, like, let's go look for the road and talk about our hair!