I hate the miss chance. It's "anti-fun," as they say in leega legun. The mechanic gives no positive feelings, only negative ones. I'm never happy when my enemy clutch misses either, I just feel bad for him.
I mean, being an 'accuracy over power' devotee myself I get where you're coming from, but at the same time, I feel that that's just kind of the point? Any time you use Fire Blast over Flamethrower, you're essentially taking a gamble for more power. If you want the extra 20 power and nigh-perfect accuracy, then you jump through hoops with stuff like Wide Lens or Hone Claws. (As was pointed out to me when I mocked using anything with Hustle a while back.)
Basically, the thinking is that if it makes a difference, the power is more important in a 1v1 competitive setting.
If both OHKO, you should probably expect a swap in many cases, but yeah, Flamethrower's better. If both 2HKO, Flamethrower's also better.
If Fire Blast OHKOs and Flamethrower 2HKOs, you should still expect a swap, but Fire Blast is better 85% of the time, and only worse after two misses in a row, which is only 2.25% of the time
If Fire Blast is a 2HKO and flamethrower is a 3HKO, Fire Blast will hit two in a row 72.25% (85%*85%) of the time, so it's better than flamethrower 72.25% of the time. It'll miss once, then hit twice, or hit-miss-hit, about 10.8% (85%*85%*15%) of the time each, so it's better than or equal to Flamethrower about 94% of the time. In the remaining 6% of the time, it's worse than Flamethrower. Easy choice for me.
If Fire Blast is a 3 hit KO and Flamethrower is a 4 hit KO, fire blast hits three in a row about 61% of the time, has one of miss-hit-hit-hit, hit-miss-hit-hit, or hit-hit-miss-hit about 9.2% of the time each, and is worse than flamethrower the remaining ~11% of the time. To me, that's still significantly better.
The numbers get better for Flamethrower as the number of hits to KO gets higher, but as the number of hits to KO increases, you usually will have either a swap you could make or a better coverage or STAB move you could use to decrease that number.