Fortune in the Middle

I’ve been thinking a lot about the skill-based spellcasting method for d20 that I discussed in an earlier post – in particular, the manner in which I suggested applying metamagic in the comments. This amounts to a sort of hybrid fortune in the middle mechanic. The idea is that you’d declare the spell, roll the dice, and depending upon the result rolled you could fail, succeed, or even add metamagic effects.

It occurred to me that this could also be used in combat in a d20 variant. What if, say, for every 5 points you exceeded the number that you needed to hit a target by you gained a success level? Success levels could then be spent on things like a free disarm attempt, adding a status effect to the target (who might get a save), or even a critical hit.

In general, I like these sorts of mechanics. They give players an interesting sort of freedom to make their character effective in the manner in which they envision them.

I’m somewhat surprised at how few conventional/popular rpgs use such mechanics, however. The only non-indie rpgs I can think of which use these sorts of mechanics are Secret of Zir’an and (in limited situations) Unknown Armies, both of which somewhat out of the mainstream. Is there a reason that these mechanics aren’t in more common use? Is it just historical momentum, or am I just a weirdo for thinking this is a good idea?