From experience, I don't feel that the Flare cards are any rarer than the Plasma cards, although this is from experience in playing TCG online.
I also believe that the Head Ringer card is classed as rare, as indicated by the star at the bottom border of the card. Having said that, the Head Ringer is a rather useful card because it's so difficult for your opponent to remove without a supporter card, which potentially wastes their turn using Xerosic when they could've used N/ Sycamore/ etc.
Should you protect it with its individual plastic pocket? No. Generally speaking, the only things worth protecting from booster packs are EXs and full arts. Otherwise, just keep it safe and that's good enough for the Flare cards.
As for its worth, I presume you'll be able to get some similarly useful tool cards (no Ace Specs), but that's about it.
Basically, the Flare cards look much rare than they are because of their design. :P