Forgive my lack of knowledge on the subject, but is a SinkEx really necessary? We already know intimately what the munitions used against this frigate can do. Why do it again?
Why not scrap her, recycle her, sell her, donate her to the Woods Hole Oceanographic Institute? Sure, maybe she becomes a marine habitat for fish, even coral, but this still seems wasteful as hell...
Because they are fun. You spend years in the military doing routine tasks and every so often you actually get a chance to shoot off a missile, fire the cannon, and blow the crap out of something. It's a big morale booster.
Every once in a while it pays to have the crews fire a live war-shot. The USAF and USN play laser tag in mock dogfights, but they will sometimes (if lucky) be selected to fire a live missile at an unmanned target aircraft. There’s a difference between a simulated engagement and the actual ‘envelope’ of a real weapon.