Many Commander staples (cards good in lots of decks) like Sol Ring or Arcane Signet are pretty cheap as singles (Sol Ring around $1.40, Arcane Signet $0.49)
Using cards to advance mana, instead of disrupting the opponent, is risky business
You don't want to overload on counterspells, but when you know the decks you expect to face in your matches, you can make sure you have the ones that'll be the most useful to include
Sure, you'll put Swords to Plowshares in just about any white deck, but unlike other formats, you can't simply splash Swords to Plowshares in your Simic deck because you want a better removal spell
Rachel Weeks of the Commander Advisory Group even suggested a template for this recently
Along with others like Dark Confidant, Meddling Mage, Shadowmage Infiltrator, these cards have been playable in competitive formats throughout Magic 's history