The film's novelisation is based on an earlier version of the script and has a very slightly different version of events (Marty drives back to his then brand-new house and hides the car in the garage rather than hiding it behind a sign) but the broad principle is sound. The car is perfectly driveable with the plutonium depleted.
His eyes fell on the dashboard readouts once again. One in particular caught his eye. It was located directly below the Plutonium Chamber, a flashing light that blinked EMPTY over and over.
Shifting into gear and moving ahead, Marty realized that did not mean he was unable to move. It simply meant—
“Good God!” he said. “What does that mean? That I won’t be able to go back?”
As to why he didn't drive around in it, Marty is smart enough to recognise that his car is an eyecatching anachronism.
Recognizing the outline of a police cruiser, Marty quickly killed the lights and turned off the radio. It would not do, of course, for him to be picked up by the police. Even forgetting the fact that he had just arrived from a different time period, he would have enormous difficulty explaining the DeLorean, plus he did not have the necessary registration papers for it or a 1955 driver’s license. He wondered what the officers would say if he showed them his 1985 license!
“I’ve got it hidden,” Marty replied. “I stashed it in a garage. It’s so flashy-looking, I couldn’t drive it around the streets without getting a lot of attention. Maybe the cops would even arrest me.”
And he obviously does drive it again at the end of the film.