Being able to exile the top two cards of your deck with the chance to play them that turn is perfectly useful. Yes, you'll burn through some cards. We would see this sort of ability useful around this time with Chandra, Pyromaster, and much later with the release of Abbot of Keral Keep - although these cards only exiled one card at a time.
Also, there were a lot of ways to make this guy truly threatening during that time in Standard. In particular, Titan's Strength, which gives a target creature +3/+1 until end of turn, could make this guy a bit ridiculous, especially considering his trample ability. Plus, Titan's Strength lets you Scry, so you can put a card you don't want to be exiled by the Flamespeaker's effect to the bottom of the deck. So, the potential to hit for 8 damage and accelerate your deck was just plain absurd.
While it wasn't quite clear at the time how exactly Prophetic Flamespeaker would play out, he was simply too good not to see play. At the time, I could imagine a red/black deck Bestowing a Herald of Torment onto him and making him a 4/6 flyer! The most likely scenario I envisioned at the time was becoming part of some Red/Green aggro deck that finds ways to consistently pump their Flamespeakers. This was during the Standard heydey of Ghor-Clan Rampager, a creature you could discard from your hand to give an attacking creature +4/+4 and trample until end of turn. Obviously, the Flamespeaker didn't need the trample, but becoming a 5/5 with double strike is extremely powerful.
Such Aggro decks would play a lot of cheap creatures anyway, so the top-deck exiling could be advantageous - especially with all the Scrying you're probably doing anyway. It's also important to note that if his exiling reveals a land card, you'll want to be sure that you can use that land that turn or you will lose it forever.
It seemed with both Chandra's Phoenix and Boros Reckoner leaving Standard that following October, there was going to be a lot of brewing going on around this card. In my opinion at the the time of his release, the guy had a bright future ahead of him. I even considered that he could be the Voice of Resurgence of the set.
None of that happened. What happened? Channel Fireball even had a Devotion to Red deck in which Prophetic Flamespeaker would play a key role. What was the problem?
The problem lies in Prophetic Flamespeaker being so fragile. He has only 1 power, and 3 toughness. Objectively, he was great. But, getting him to actually activate his ability could prove difficult. Channel Fireball also admitted at the time that he wouldn't make it in Modern with Lightning Bolt running amok.
Despite being relatively fragile, Prophetic Flamespeaker did see some Top 8 play during his time in Standard. On Magic Online, he popped up in some Red Deck Wins lists in Theros Block Constructed. He also popped up as a playset in a winning Jund list in a Magic Online Modern Daily event, although this appeared to be a fluke. It would pop up in some top 8 lists at State Championships in Boros decks, but for the most part, it wasn't a card that really did as much many players hoped it would.
Later on, Prophetic Flamespeaker made another appearance in Modern as part of an All-in Red deck that had a strong finish at a Star City Games Modern event in Las Vegas in December of 2015. Then, the Flamespeaker would appear in a couple of Dragon Stompy Legacy decks in April of 2016 as a three-of in the main deck. In fact, in one event, two similar lists placed first and second. Armed with Sword of Fire and Ice and/or Umezawa's Jitte, the Flamespeaker actually was able to get in some attacks. Then, outside of popping up in some Commander lists, the Flamespeaker basically disappeared from view.
Prophetic Flamespeaker is the perfect example of an objectively powerful card that simply never found the legs to be consistently part of a competitive strategy. He would latch on as a part of existing decks, and while the deck would win, it seems that he didn't actually improve the deck enough to become a permanent fixture. With powerful creatures continuing to be printed all the time, there just isn't really a slot for the Flamespeaker to fit into anymore.
Still, Prophetic Flamespeaker is a good creature with the ability to potentially finish off an opponent with the right pump spells. He also can provide some powerful card advantage if the deck is able to support casting spells quickly. Prophetic Flamespeaker was indeed powerful on paper, but in practice, he never really panned out competitively. For a card that's worth roughly a dollar, he's a fun creature to try out and brew with in Kitchen Table Magic decks. Sadly, many people have forgotten about him.