Without supply you'll be stuck at one speed, even if your engines don't require supply.
Its a safety net so you don't design useless fighters.
Only one fighter drive is effective per vehicle.
However, the more powerful engines are also larger.
You will commonly wish to use an older, smaller drive on your heavy bombers in order to cram another missile launcher on them.
The energy weapons do not use supply, but have a correspondingly massive cost in radioactives to power them.
They are a bit weaker and more expensive, but will be pretty much required for any extended sortie into enemy space.
Meanwhile, if you are sitting on a resupply depot, you are free to expend the cheap and powerful ordnance without worring about running dry.
