He likes it. I think that's a big part of it.
The Hofner was pretty much retired until the late 1980's when Paul McCartney teamed up with Elvis Costello for some songwriting/performing. They got around to recording an Elvis Costello song called "Veronica" that features Macca on bass, and Elvis Costello said that he thought that part would sound great on the Hofner 500/1. McCartney initially objected, saying that it had real problems staying in tune, etc. But they got his Hofner fixed up at Mandolin Brothers in New York, and it wound up playing much better. As a result the Hofner wound up on that Elvis Costello record, and it seems that Paul rediscovered how much he loved playing that bass. The light weight and the short scale made it very easy for him to play that instrument live. The Hofner also wound up being featured on some songs on Paul's Flowers In The Dirt album that also featured some collaborations with Elvis Costello. One of the notable tracks featuring the Hofner was "My brave face". Although there were other tunes on the record that were played on a Wal 5 string like "Rough ride" and "Figure of 8".
And you know what else? Macca is very much concious of image, too. And I think he recognizes just how much he is personally identified with that Hofner violin bass.