IMO, your passport is the best ID available. It shows your picture and name and is a universally recognizable form of positive ID. Your drivers license is best tucked away in your wallet.
Should you "lose" your DL as you whip it out at various checkpoints, you're SOL when you get to your destination and need to rent a car. Of course you're in a similar unfortunate position if you lose your passport when traveling internationally. But most of us travel far more domestically than internationally so it stands to reason to protect the DL over the passport.
And for those of us who have had experience dealing with the DMV, replacing a passport is simpler than replacing a DL.