What if your team spent two months meticulously documenting each and every use case for your product? Specifying each and every interaction users will have with the product to achieve their functional goals?
What if, in addition, your team spent two months meticulously documenting what the user interface will be. Wire frames, screen shots, click-action-response tables, etc.?
Would you end up with a usable product?
Not necessarily. The old adage is that you don't improve what you don't measure. You can specify how a user will use a product all you want, but if you don't define what "usability" means - if you don't measure it- you may not end up with a usable product.
Invest some time defining the usability metrics. How long should it take for newcomer to learn how to use the product? How long should it take for an experienced user to achieve her functional goals? How many user gestures should it take?
Invest some time testing usability. Have your testing team put together a suite of tests that verify the usability of the system against the metrics you defined. Include the key end-to-end usage scenarios that deliver value to the user.
Okay, maybe functional specifications don't themselves jeopardize usability. But spending all your team's time on the functionality of the system without defining usability metrics and testing your product against them is not likely to lead to a usable product.