Are devils worth 7 blood each?

[Huzurdaddi]

The soul contract can never pay for another soul contract for free (as is the case for the 20-turn clam with alchemy. This is a very important difference.

I agree with that. 100% correct.

Soul contracts get you devils, which are only good for flying around and smashing things. Granted this a pretty generally useful ability, but not nearly as flexible as a high astral income.

Not as flexible true. However the soul contract produces the end product. You need not spend the caster time ( which is important ) to convert your gems into something that can smash things.

I like devils a lot, but I haven’t found soul contracts are a good investment in a close run game. I’d rather have my devils right now, keeping me alive, or killing off the other guy.

I would imagine that in close game you are spending your blood slaves on SC devils not on devils. It's very hard to actually summon devils in any quanity ( except once you reach blood 9 ).