You can’t honestly tell me you haven’t thought this, if you’ve played this series for more than a few minutes. It’s a valid question, after all, why would anyone use a grenade that takes ~5 seconds to go off in a game where you can die 10 times over before then? Well, it has a very specific purpose, just one that’s also filled by other, better choices of equipment.
When you see that guy, sitting in a window across the map, firing at everything that moves? That’s when you use it. You can, with a little practice, have a grenade go off right in his face. Often without him ever being able to see you. We always complain about campers in this game, so why would we pass up a way to remove them? The other options are just better.
The semtex and C4 are simply superior for offensive play. None of this “hold R2 for 2-3 seconds and hope your target is still there and pray you haven’t been shot in the meantime” nonsense. Just throw and maybe tap square twice. Easy. Launchers are even easier. No timer, no risk of your grenade rolling away. Just press a button, and someone explodes. And these other options can be reasonably used in the fast-paced combat the series is known for, not just for removing some guy who decided to sit still for a while.