As the only 0-cost bloon alongside Swarm Red Bloon, the Red Bloons are useful for rushing against the opponent on turn 1, when they will likely be uncontested. Having large amounts of them can be a considerable threat if the opponent lacks Spike Storm! or Firestorm.
However, note that opponent will have 4 Gold (P1) or 5 Gold (P2) by the time that Reds become a real threat, enough to afford two Spike Storm!, Firestorm, or at least 2 cards worth of Dart Monkey Twins.
If using Bloon Embiggen or Growth Gas Bloon, saving Red Bloons for buffing them later may be worth doing if the opponent's field clear is not a concern.