Because the Swit lasts just as long, gives accurate in-camera feedback plus charge-check capability when not attached, has a DC-out terminal (to power light, monitor, or field recorder), and costs about $60 less than the Panasonic.
It isn't necessarily going to be one of the Ebay batteries. I've had good luck with them for the HVX, usually paying about $20 and getting 2-3 years of life before they start losing capacity. I bought one for the AF100 and after one use, it could no longer power the camera .