Not me but my older brother who used my copy of Spore before me did use cell mouths the whole game! It's not a big deal, just make sure you have other self-defence or social features to cover up your Level 1 Sing/Bite lmao.
This may not be what you are looking for, but I thought I'd share my experience. I didn't use the cell mouth as the only mouth, but I kept the omnivore mouth from the cell stage.
Despite my efforts of varying my cell's diet (and I've tried many times), the game still classified my cell an "herbivore" at the end of my cell run, so my mouth choices if I upgraded were only herbivore mouths. So what I did was keep the omnivore mouth on either side of an herbivore mouth. As long as I have those, my creature gets to stay an omnivore. When I experimented and took it off... herbivore. So I hit "undo" and kept that omnivore mouth. Interestingly, my "special ability" was still the herbivore one (Siren song) despite my label saying "omnivore."
And when my creature eats, it eats with one of those omnivore mouths. It sings and bites with the herbivore mouth though. So far, even in the Tribal stage, the tribe members can collect killed animals as food, so the creature still seems to function as an omnivore.