Well....I like both Electabuzz and Electivire, so you bet I'm going to get two Electabuzz and only evolve one....
But at the moment, I seriously dislike the thought of evolving my Buizel! He's just...well....cuter and cooler than Floatzel at the same time....I don't know what it is about Floatzel, I know he's scary, but so's Torterra to some people, but I don't like him that much...sadly, it means I never get Crunch or Ice Fang, although I can tutor Ice Punch.
I also tend not to evolve Quilava until I really can't win without the boost. It's not that I dislike Typhlosion, I just prefer Quilava, so once it's fully evolved, I always get a couple of eggs and keep a Cyndaquil and Quilava.