Do you evolve your Eevee as soon as possible, or after it learns a specific move or reaches a specific level? Have you ever kept one as an Eevee?
I mostly like to evolve my Eevee as soon as I can, especially the ones that evolve through stones like Vaporeon and Jolteon. I don't like waiting for moves as an Eevee because I might miss a good move the Eeveelution learns if I don't evolve it. I did leave an Eevee as it was, but that was for dex filler.