IMO It depends on whether you have a baby that thinks the cot is a circus (as my Jacob does). At times, he insists on lying vertically in it, as opposed to horizontally and he was starting to bump his head on the cot bars and get his legs stuck in between them too.
I bought a cot bumper that goes all the way around the cot. Its firmly attached, so cant come off or be lifted up by him. Since I put the bumper in, we've had no bumpy head/ stuck leg incidents.
I also like the look of them as his one matches all the other stuff in his nursery (Blue Jean Teddy theme).
Got mine off ebay..cost about £8.
The only downside of my one is that I cant see him from another room through the cot bars (and vice versa). But overall, Im glad we have one. When he's a bit older it will come off of course.