While it certainly is a cool idea to have a flaming sword, it seams not very practical for a wizard to do something like this. Even if you created such a rote, using it would cost one point of mental stress and your standard action in the exchange, only so that you are enabled to attack with a weapons skill, that in most cases will not be as strong as your conviction or discipline ...
Instead of boosting his/hers weapon every wizard that is after inflicting damage would simply go with two normal attack evocations. First of all because they will deal far more damage and secondly because they are much more likely to hit the target.
That said, the rote like it is described seems not to far out. It certainly gives some nice benefits, but at the cost of actions and mental stress, that could used in other ways to similar or greater effects. After some testing to see how it works out in game i'd probably allow it if a player really would like to have such a rote, and if it's just for the coolness of it...
For a minor practitioner of cause I can totally see some benefits, that such a rote might offer as they are more limited in what they can do...