Why would I only use an Eevee? I want to box it and use the other Pokemon to beat the storyline. If I find a Fire Stone or Water Stone that I can use, I could evolve it with one of them, and not worry about that Eevee evolving into Espeon.
The Eevee I had is actually Bill's Eevee in Leaf Green. Since you can't evolve Eevee in Leaf Green into Espeon, I traded it to Emerald, but it has been difficult to get it evolved quickly.
Edit: I could have misunderstood you. Are you saying that I will not have Eevee evolved the whole time, or are you saying that Eevee is the only Pokemon I would be able to use?