Venting flux is supposed to make your ship vulnerable, it's the cost of taking heavy shield damage and firing your weapons a bunch. If you could vent flux for a speed boost, or have it destroy incoming missiles, it would remove that vulnerability.
No, that's not what I'm suggesting. I feel like I was pretty clear about normal venting only doing very minor damage, especially in the case where I specified that it MIGHT destroy one missile in twenty. As an alternative, it could merely slightly push at projectiles so it might only make a missile with an already awkward trajectory veer off and miss its first pass.
As for specifically using flux as a focused weapon or speed boost: in the case of being weaponized, it would still be of only marginal power, either dealing very small damage or just physically repulsing, and in the case of speed it would be VERY short lived, and both cases would require a customized loadout with systems just to enable them that take up space and/or ordnance points.
It's really not difficult to imagine any number of ways to create what I'm envisioning here that is still balanced with respect to the intended weaknesses of flux buildup. Flux buildup already empowers your energy weapons after you reach a certain point, you know.
The real question is: would this be fun and interesting enough to warrant consideration?