As a pianist first and foremost, a LOT of my bread-and-butter/earning-money gigs required me to cover bass parts. The trio I gigged full time with for two years (back in the early 80s) was me on keyboards, a drummer, and a horn player (and we all sang), so I was playing keyboard bass 6 nights a week at least. I actually used to enjoy doing that very much.. at the time I was using a MicroMoog for bass, a Yamaha PF something or other followed by a Korg SGD for the piano sounds, and a PolyMoog followed by a Korg Poly6 for horn, string, and synth hits and pads. I loved the fat sound of the MicroMoog and the ability to grab a knob and adjust/tweak stuff on the fly (although I didn't enjoy it pitch-drifting with no notice... I learned to play in two different keys quickly!). I started doing left hand bass after being inspired by various Herbie Hancock and Chick Corea LPs I had, and one of my college classmates used a MicroMoog with his keys/guit/drums trio to cover bass. He was great at it, and I figured I wanted to do that, too.
Fast forward - still have great respect for B3 players; that's the real deal. Nowadays, however, I wrestle with the guilt I have playing LH bass and putting a bassist out of work, especially in the past couple years as I've become more serious about being an actual bassist. While it's nice, as a keyboard player, knowing that the bassist and the pianist (when both are me at the same time) will ALWAYS be playing the same changes, it's not the same as having another musician to feed off of, and just feels a bit more sterile and contrived compared to playing with a great bassist. On the other hand, I'd rather play LH bass than work with a mediocre bassist; nothing kills a gig like a bassist playing wrong changes, getting lost, or floundering around the beat.
Economics are economics, I guess. Not sure if I'm addressing your question or not, actually. Let me try again, more succinctly....
"Have you played keyboard bass instead of bass guitar?" - yes, extensively.
"Why?" - Initially because I thought it was cool and fun, but mostly because of economics the past three or so decades.
"What did you think of it?" - I MUCH prefer a real bass and a real bassist, as long as they are very good players and musicians. I'd rather play LH bass than play with a less-than-professionally-competent bassist. I'd NEVER play a gig playing just a bass keyboard; if I'm not tasked with playing other keyboards, too, I'm playing a real bass.