And now demonstrating my ignorance of suspension systems, what is the function of those plastic bushings? Is it to keep lubricants above them? Some other purpose?
The roadster came with foam bushings for the suspension which are now disintegrated so really there are no bushings on the roadster. If you hit a pothole or slam down on something hard you could bottom the car out if it hits the metal on metal if nothing is protecting it. The black springs are very durable for heavier cars but anything is a possibility; rather be safe than sorry.
Since rubber bushings in your suspension take a pounding any time your vehicle is in motion, they must be extremely robust and durable in order to last. And, generally, they do last for quite a while under most normal driving conditions - so much so that automakers don't specify any kind of replacement interval. However, rubber bushings in your suspension do eventually succumb to wear, tear, and dry-rot over time. When one or more suspension bushings give up the ghost, you've got metal-on-metal contact creating noise, vibration, and harshness. more details on -
When Is It Time To Replace My Suspension Bushings?
Google result - To be blunt,
no, it's
not safe to
drive a vehicle that have any
bushings that are completely
broken. ... When they fail, those components
can snap and cause serious damage to the vehicle, but more importantly, it
will create a serious safety hazard for you and any occupants in the vehicle
The bushings were the first thing I noticed that were disintegrated upon taking off the belly panel when I first received the car.