As Keith said, it depends. Early on, the quarter inch jacks were stereo, later mono, later with stereo/mono switches. Some were custom ordered one way or another. Some were later modified with help from the mothership one way or another.
If I recall correctly, if it's a mono jack, then both pickups are wired together into a single signal. If it's a stereo jack and you plug a regular mono cable into it, then yes, you only get one pickup. As Toby suggests, the solution here is to make, or acquire, the stereo to mono cable that Mica describes on the linked page.
Of course if you are out at an open mic night and you have a stereo to stereo cable, the thing to do is to plug the neck pickup side into channel one of the bass amp, and then plug the bridge pickup side into the guitar amp on the other side of the stage; then tell the guitar player to sit this one out, you've got it covered.
