I think you dramatically underestimate the amount of effort required to produce a formal model of any nontrivial system, and overestimate the stability of system definitions in response to changing business requirements.
The sort of things for which you write specs are generally "core" functionality that doesn't change very often. If people are constantly mucking around in that area without writing specs, your system will be a pile of crap.
That discovery is actionable, many other bug discoveries are not without further investigation.
[0] This statement should be qualified with: Especially if it's at all novel to you, your team, your organization, or the world. And the amount of time to spend varies based on its true novelty. A team that's made hundreds of RoR sites can churn a new one out in a week with minimal upfront design effort. A team that's making the next RoR should probably spend more time thinking about what that means before making, or committing code to, it.