This is an old favorite card of mine, when I had a legacy burn deck. I don't think it's great for Cube, though.
Generally speaking I think it's actually cool to have a few super-narrow cards, but I like them to play a defining role in deck archetypes. It can be really fun if you can change the way the game is played by just resolving a single spell. Sulfuric Vortex comes to mind. Vortex can play a really key role in burn and some aggro decks. Flame Rift I feel doesn't quite define a burn deck - it's just sorta one more card to help you hit critical mass. But, you can do almost as well with Searing Spear, a much more versatile card that plenty more drafters will be happy to see.
No two cubes are the same, and maybe Rift can be fun to include in your list ... but I would caution you that if you include a lot of cards that are super narrow and yet kinda replaceable, you might have a number of drafters ending up with piles of unsatisfying cards.
Yeah the reason cards like Vortex and Goblin Guide and Fireblast are cubable, even though they are so narrow, is because they are the best at what they do by a long margin. Flame Rift is not really substantially better than all the bolt and 2cmc bolt variants even in the most aggro/burn of aggro/burn decks, and it is completely unplayable in any other deck. So while it is a good effect in those decks, it's not worth the cube slot cause it doesn't make those decks much better and it will never be played anywhere else.
right on, thanks for your thoughts - I saw that 4-face-damage-for-2-mana and got excited, but y'all make good points that the one extra point of damage vs searing spear etc. isn't worth the huge loss in both flexibility and in number of decks that would play the card.