What Loeb did in Ultimatum, I think, was to show war and death in a more realistic way, in that, amidst carnage and chaos, attrition isn't always reflected upon deeply until afterwards (a requiem, if you will). It's more like action and reaction. Honestly, I hated Ultimatum, but I definitely see where he was going with that.
As far as the Secret Invasion situation, it tied in pretty well. Janet was made an interesting character for a moment, and sometimes it takes losing a character (or, you know, a person...) to realize just how important he/she/it was. Her death was the motivation for the new (and I'm going to say "better") Mighty Avengers. Her death was a catalyst for Hank getting his head out of his ass. I'm not going to classify her as a "refrigerator" girl, either. She wasn't some secondary or tertiary character; a girlfriend, relative, or whatnot. She was a hero, and heroism means putting yourself into a situation when your nerve out-paces the potential consequences, even if that potential becomes a reality.
Ultimately, we're all going to die at one point, and it doesn't matter if we're loved, despised, or ignored. In her death, Marvel and it's creators are granted an opportunity to spring forth new life in a lot of what affects the Marvel Universe.