Being a DB/DBZ/(die GT) fan, I'm ok with the miracle hidden power/technique in many ways, I guess deep down inside one is waiting for a miracle to happen, I mean, is the hero really gonna lose/die just like that?, something's gonna happen and the hero is gonna beat that antagonist's ass.... and when it doesn't happen you're like wut o.o . . .
But if he looses, when he finally wins is sooo much rewarding =].
Also, It depends on the kind of story, a dbz kinda story you really need to have that miracle power, that's the part when peeps get excited, we gotta remember the target audience of certain works, if you show a kid a deus ex machina development, probably s/he would love it and get all pumped up, so you can't really take it away.
So I guess target audience may play a role in this, give the character dies scenario to a bunch of kidz and you probably gonna fail (and if s/he really dies then have the miracle happen afterwards and have the character being revived by a magic potion, example SPOILER ALERT: Sailor Moon R: The movie), who knows. So i don't think that 100% of the time is the result of poor writing, and I'm ok with it most of the times depending on the type of story like I said XD.
P.S: My favorite is the power of love 