Probably not for the first few months, they don't move around much. Once baby starts rolling and trying to crawl you might think about the mesh ones, but it depends on your child. Some kids get their arms and legs through the bars, others don't.
As for bumping heads on the sides of the cot - well, it's not impossible but it's incredibly unlikely that they would be able to hit themselves hard enough to cause any harm.
I wouldn't use traditional bumpers, personally. The only real advantage I can see is that they look nice, and although the odds are very small, I'd blame myself forever if baby died.