WannaBe, in my case because when I was actually in that situation and had experienced both A. falling into a deep exhausted sleep in an uncontrolled way with a baby on my lap and B. falling asleep in a controlled way with a baby near to me on a firm mattress, in their own grobag, with lots of space, and without the exhaustion that comes with having to get up several times a night (so I slept more lightly), it became blindingly obvious to me that (for us, with our setup), B was safer.
That didn't mean that I didn't spend a lot of time paying attention to safety and instead just carelessly thought, sod it we're co-sleeping, I'll just bung him in the bed anywhere. Depending on the age of the baby my preferred distance for sleep rather than feeding could be a good foot away, with just my arm out to touch them. It was a big bed! I was also obsessive about room temperature, air quality, mattress quality and clean sheets. Not because we co-slept, I would have been anyway - just illustrating that it's possible to be a bit obsessive about safety and still co-sleep!
There are many ways to co-sleep, and the way people deal with the guidelines issue is to look at it closely and make a judgement. The guidelines are the reason why if you do have a good reason to do it then you take precautions but just as with formula and bottles they're not in themselves enough of a reason for no one to do it ever.
Also it's become clear from this thread that some things (baby right next to you in a bedside cot) are counted as co-sleeping by some people but not others - see I would count that as co-sleeping, an example of a way to do it safely, others seem to count that as not co-sleeping, so narrowing the definition of co-sleeping to exclude one of the safest ways to do it.
I think what would change my attitude would be if the risk from co-sleeping was purely SIDS, rather than smothering and suffocation. The SIDS risk is why I would be far more afraid of putting a baby into a room alone early on. There's nothing practically you can do in that situation to mitigate the risks, because no one knows exactly what it is about being in that situation that's dangerous. But with co-sleeping you can take practical steps to make it safe, because we know what the main risks are and they are practical ones to do with position and heat and bedding and parents not being aware.
Combine that with the idea that co-sleeping may help a baby to regulate their breathing, and the fact that the studies the guidelines are based on are iffy when it comes to how well they looked at safe co-sleeping rather than unsafe co-sleeping or unplanned bedsharing, and it's really not hard to see why those guidelines aren't enough to drive everyone away from doing it.