OK, that's interesting. In your original post, it sounded like the keyboardist was being an arrogant tool, trying to dictate to the band. But having read this, I think he could have just offered it as a (badly worded?) suggestion. If he's being a control freak then I'd be worried, but it sounds like the band has something interesting going, so I'd investigate further.
Also, now we know more about the concept of how it will be recorded. It sounds like the band is fairly collaborative - you should collaborate on the recording process too, at least to be aware of what's going on and how it's going down. Last thing you want to do is end up leaving the keyboardist alone at the end and then lose control of the project. (I could be totally misreading this, I'm traumatized, read one of my previous posts.) Is the drummer going to record the drums out of the blue? With a click? Couldn't you be there playing along in the headphones even if what you're playing may not make it to the final recording? Heck, record it anyway, you might come up with some good takes. If this is the route you took, then I think it makes sense to use one bass and bash them out!!! Then, since the original idea was to overdub bass anyway, take a listen and maybe change stuff, redo entirely with another bass, whatever works.
There are very few absolutes in recording - there might be "accepted wisdom" or "common knowledge" but often the interesting stuff comes from not following those kinds of "rules." Learning that you'll be overdubbing anyway opens up a lot of possibilities in my mind.
TL;DR - Get the basics done, then experiment. And get everyone on the same page.