Whoever said they couldn't get out of bed without stepping on the baby's head - unless you literally have to make your way to and from your bed along a path that is the width of your palm, and your bed is surrounded by walls on all four sides - that must be a slight exaggeration, surely? Also, if there is room for a double bed, there IS room for a baby.
I guess this is just one of those where people choose to make up their minds, and stick to it, whatever the evidence-based research says.
Some people smoke and don't get cancer. Some people don't smoke and they still get cancer. Years ago we weren't aware that smoking causes cancer, then we found out that it does. The official guidelines changed - that doesn't mean that you might as well smoke because people used to say it's harmless, surely?
The way I look at it is that although the risk of SIDS is small, why would I do something that is proven to DOUBLE the risk? Yes, it's still a small risk, but as someone else pointed out, there is no going back.
It's very well to say "what are the chances of one of the parents being awake if the child stops breathing? Next to none, so no point sharing a room..." (paraphrasing slightly) but if you actually look at the research, it's not about one of the parents possibly being awake - it's about the parent's breathing and moving in the same room which keeps the baby from sleeping TOO deeply.
If you actually cosleep and share a bed, the human body is amazing: if the baby gets too hot, the mother's body will cool it down. If the baby gets too cold, the mother's body will warm it. The mother's heartbeat keeps the baby's heart beating steadily, the mother's breath keeps the baby from getting apnea, or at least reduces the chance. Also, the mother's and the baby's sleep cycles end up synchronising, so that when the baby wakes, the mother will be in a light sleep cycle too - meaning that the mother isn't having to rouse from a deep sleep, which in turn means that she will wake up easily and quickly (as opposed to having to drag herself out of a deep sleep, out of bed, across the hall...)
Knowing all that, if someone STILL chooses to put a baby in a room of it's own, then I suppose they have weighed the evidence, and their priorities, and made an informed choice. But people should make sure they are making a choice based on research-based evidence, not based on "never did me any harm / sounds pointless".