I hear you - ideally, you'd have perfect control over both. The question is, do the scenarios in which you actually want to have this control occur often enough to justify making it more cumbersome all around? Having an extra button for this type of shield control... I'm just not sold on that yet.
How about something simpler than my original idea: a shield lock button, which toggles shield movement on and off? Doesn't add any cumbersome-ness; without using it, the shield behavior is exactly as it is now (with the toggle, anyway). I'm growing more and more convinced of the need for this the more I play with my high-intensity-laser broadside Conquest--things I'm hitting with the broadsides are generally a long way off, and in most cases the threats I'm having to deal with are from a different angle.
I can also see it coming up with a frigate with omnidirectional shields fighting a bigger ship, a la Sinking the Bis'mar. Missiles tend to come at you from the sides in that mission, simply because they have to chase you, and the same situation might come up elsewhere. Being able to lock shields to face missiles while you make a strafing firing pass with bigger weapons seems like a pretty good motivation for lockable shields, too.
