Think about it; would you want your body to transform itself in a way that isn't reversible? If you don't like what it changes into, you're rather stuck.

[[Really, it's a rather scary thought. One of her Pokemon is coming close to his evolution level and she's starting to worry.]]

Although I have read that Pokemon who delay their evolution will have a more varied movepool than those who don't, so I suppose you could consider that somewhat of a perk.