Sorry, what is an assault weapon? Do you mean full auto assault rifle or do you mean semi-automatic rifle that looks like an assault rifle?
Full auto assault rifle has been heavily regulated ever since National Firearm Act of 1934. It's so hard to obtain nowadays, the cost is typically in the 10K range.
If you mean AR15 type semi-automatic rifle that looks like an assault rifle, I would say that if you are rational and a believer of self-defense, you would need one, and here are the reasons:
1. It saves lives. Since someone said if it saves one child's life, it's worth the effort to ban, then if it saves one child's life, it's worth having one.
Here's the proof how it saved a child's life:
https://www.youtube.com/watch?v=DuhKCiY-lu0
2. You don't want to be outgunned by the bad guys. If a typical bad guys bring pistols to rape and kill your family, what should you do? You bring a rifle. Since you can't afford full auto, a semi-auto lookalike would suffice.
Well, even they come with their crawl bar, you should still have a rifle - why pick a tool that is less effective?
3. AR15 is lightweight and easy to learn how to shoot accurately so that it's less likely you would shoot a good guy by accident.
4. Contrary to many, AR15 actually fires a less potent round than most deer rifle. It has much less penetration than a 9mm pistol round. At home, it's a fantastic choice for self-defense because you don't have to worry about a strayed bullet wonders into your neighbors house.
5. It takes 30 rd magazines so that it's hard to run out ammo even dealing with 4-6 bad guys at one time.
6. It has adjustable stock so that you and your wife or daughter can share one gun instead of two.
Watch this one and tell me in that situation, you wouldn't want an AR15 with 30 rd magazines.
https://www.youtube.com/watch?v=DuhKCiY-lu0