After they literally blasted Tirek to hell, we haven't seen the Mane 6 use Rainbow Power again, except in Luna's dream sequence.
So can they activate Rainbow Power again at any time they wish in the event that a new, powerful, irredeemable enemy comes along, or is Equestria left virtually defenseless if, say, Chrysalis is still alive (no need to debate that, her fate remains ambiguous and is up for us to decide unless they decide to bring her back), or someone breaks Tirek out of Tartarus and gives him his power back, or a new villain (*coughcosmoscough*) comes along. Unless of course they lure them to the Crystal Empire and use the Crystal Heart to destroy them just like it did Sombra and that snowstorm, but they never thought of doing that with Tirek.
So, suppose Chrysalis had attacked again before Tirek began stealing ponies' magic, and the Mane 6 had used Rainbow Power then to blast her to Tartarus (because blasting her over a cliff again wouldn't work as her return would prove she is immune to long falls). If Rainbow Power was a one-time thing, then we'd get the fourth bad future seen in The Cutie Remark.
Plus, I'd like to think that the Mane 6 defeated the Bugbear with Rainbow Power, since they didn't seem able to finish him before, unless you think Rainbow Dash Rain Nuked him or Fluttershy used The Stare to force him back into Tartarus through the gate. I think they either used Rainbow Power or killed him in a way too violent to be shown.
This is debated on Chrysalis's article which nearly led to another status debate so I simply said key points for newcomers to hear as to why it would be appropriate to kill Chrysalis in the show, then changed the subject back to the original post, but now that the Rainbow Power issue has come up I think that is what should be discussed now.