For Those who are wondering what their bile is for, check the wiki. It's the irreplaceable material for crafting pheromone darts, the dart can be used to attract all wildlife around the target to enrage and attack the target for 30 seconds. Useful in pvp but just about useless in singleplayer. So if you're on singleplayer (like me) there's not much to gain by killing ammonites.

Some things that i don't think anyone has mentioned yet:

1. Ammonite rage attracts not just normal wildlife but alphas as well, and as people have said they will not be restricted by their normal limit in water so Look around before hitting an ammonite, especially if u can't kill it in one hit.

2. As Icthyosaurus have a "invisible" glitch/feature that causes creatures to simply ignore it and it's rider most of the time, even when attacked.(shared by all creatures tamed inside tribute caves. To ensure you do not anger any creature u want to kill hit from behind and wait for the creature to calm down before hitting again if it angers.) It might seem perfectly suited to kill ammonites. Wrong. ammonite rage completely nullifies the invisible glitch when it's active, often resulting in a breathtaking sprint away from furious plesios, mosas, and in one occasion an alpha mosa that nearly managed to kill my icthy. So Unless you just want a quick getaway mount, don't bring an icthy to kill ammonites.

3. Aside from their unique bile, they also drop silica pearls, oil, and rarely black pearls. And they're not as rare as some of the other black pearl dropping creatures. So If u can handle anything the ocean throws at u it might be worth your time to find and kill them for the pearls.