I buy what I think looks cool, and then build armies that I think will be fun to field. My first army was Sisters of Battle (not Witchhunters). I had a blast playing, and could hold my own against everyone in my group. If I didn't win, I made my opponent work for it. Well, except for one game where I made a deployment mistake and got gunned down by my opponent's IG.
I play to have fun, put cool models on the table, and hopefully win. Rules are good, but the Rule of Cool is more important.