But mana rocks are also very good. Many of them are mana ramp and color fixing all at once.
I would argue that rocks are better than mana dorks, because 1) you have black / red / green / white removal for mana dorks (and blue can counter anything, but it's time-critical), versus only three-color removal for rocks (and white doesn't remove artifacts in every Standard; also, artifact removal is more often a sideboard thing than a regular thing), and 2) borderposts notwithstanding, you can fix colorless to colored mana with rocks. What I mean is, to use an example, if I play Avacyn's Pilgrim , I go from monogreen to green/white. If I play a Selesnya Signet , I go from monogreen to green/white, or from monowhite to green/white. Costs more, but isn't exactly a huge amount.