personally, i haven't got problem playing 1 drop or two drop white spell with 4 scrubland, 1 godless shrine and 2 plains(and fletchland that can provide plain : 2). Maybe am i lucky, but i've never tell myself "damn, i need one more white mana, should have play mox diamon".
And about the argument that you do your 3 cost spell before your 2 cost spell, i think it's just better. Cause if you do your 3 cost spell in the begining of the game, then you have ton of mana to disrupt your enemy.
Finally, i've never feel to need a mox diamon in my opening hand, contrary to dark ritual.

Holo.