Wow, this has to be one of the most misogynistic comments I've ever seen on Slashdot - and that's saying a lot.
Here, I will pretend like you're not a total douche, merely ignorant, and try to explain things politely:
The implant is hormonal birth control. Many women cannot take HBC, or only some HBC, due to extreme side effects such as depression or mood swings, weight gain, and heavy bleeding. Only some women stop getting their period on the implant - up to 20% actually have heavier periods than before. Also, HBC puts you at higher risk of clotting problems (such as heart attacks, strokes, and embolisms), which means that women with other risk factors may want to avoid it. And women on certain medications, such as anti-epileptics, can't use the implant.
Other women may simply prefer other forms of birth control for other reasons. For example, some women actually appreciate getting a "Hey, you're still not pregnant" reminder every month. Some are uncomfortable with getting something implanted in their body. While their preferences may inconvenience you, it is far from "negligent" for them to make that decision for themselves.
Perhaps if you feel you are having to "endure" your significant other, you should let her know that. In those exact words. I'm sure she'll be refreshed by your honesty and see you in a completely new light, and will happily rearrange her biology for your convenience.