While an empty GEM-60 is just under 2 tons, it occurred to me that additional thrust could be generated by jettisoning the empty boosters (or a large portion of them) BEHIND the Delta IV rather than pushing them off to the side. Is there a reason we don't configure rockets this way?
Because the extra thrust would be puny comparing to the rest. You'd gain 1-2m/s if you're really lucky...
Because you really, really don't want the booster to crash into the rocket, the bell nozzle, or anything else, and it's subjected to massive aerodynamic forces that could easily do this if you don't push it away strongly enough.
Any seasoned Kerbal Space Program player will tell you a dozen stories of boosters on their rockets destroying the rocket, or the engine, or ripping the fins off, or misbehaving in similar manner, because separation didn't throw them away hard enough, or other tricks, like fins angled away from the rocket, spinning for centrifugal force to eject the boosters, separator SRBs or similar methods failed. And actual rocket scientists will just nod their heads. These boosters are heavy, and they can really mess things up if you don't make absolutely sure they won't.
Never mind the mass of such propulsion mechanism would likely be greater than mass of equivalent amount of fuel you could pack up and use at superior efficiency in your rocket engine.