I can only speak for myself. When I was called to this church, I was a (very rusty) bass player. We had keys, bass, and drums, an occasional trumpet. About 6 months later, I bought a 5-string bass.
All along, I wanted something (other than harmonica) for slow worship music. My granddaugher (who lives with us) started taking piano at school, and I bought a stage piano for practice. Then my wife and I started classical piano at the local community college.
I bought an older Kurzweil K2000VP, and started using it along with the bass at church (some songs on one, some on the other). Gradually, I found myself doing more of the bass on the keyboard, especially after I bought the Bass Gallery CD with a lot of very good samples. Now, I have the K2000 and a PC2 at the church. As I get better in playing, I find more cases where I have the bass part running on the 2000 and something like Hammond/Leslie right hand on the PC2.
I also do a lot of orchestral parts in worship music. I have noticed that I almost exclusively use those instruments with which I have a good knowledge of HOW they should sound. In orther words, when I'm playing bass, I'm pretty much playing the same patterns, riffs, and little extra tweaks (hammer ons, slap, etc) regardless of whether I'm on bass or keys.
Right now, our group has a lead keyboardist, drummer, two guitarists, trumpet, and myself. My particular part in the group is adding the filler to make the sound fuller and more balanced - whatever it takes to do that. I've almost completely stopped playing either real bass - my hands hurt more (I'm 66).
For a little while, we did have a bassist, but he moved on. I NEVER stepped on his place while he was there - it freed me up more do do other things.
Jim