That's a good point, but system complexity and failure risk is also a liability that needs to be considered. A few meters of ice around your ship will be pretty

That's a good point, but system complexity and failure risk is also a liability that needs to be considered. A few meters of ice around your ship will be pretty much 100% reliable and require zero power, but for the considerable extra fuel required to achieve your required delta-vee. An ice-hull will also stop micrometeorites which I'd imagine a plasma shield would not be able to deflect. Space is hard!