Depending on their visual condition and ability to adapt with it, 20/200 vision is indeed limited, but can mean having a LOT of functional vision: sometimes, not enough to see a step before you fall down it, but with other conditions (like achromatopsia, which I've seen bioptic driving used with) having enough vision to read a regular-print book without any sort of vision aids.
20/200 vision means that, with the best ordinary correcting visual aids, the person sees at 20 feet what the 'average' (which is not to say the best healthy) eye will see at 200 feet, in terms of quality and richness of visual information.
However, looking through a bioptic telescope is not considered an ordinary visual aid because you can only spot through them, not use them for sustained periods of time. For the purposes of spotting street signs and road signals, the person is still required to get the level of vision required for ordinary drivers (which is 20/50 for a regular license and up to 20/70 for a daytime restricted one, here) and there's usually a legal limitation regarding how low-strength the telescopes need to be.
There has been no proof, as of yet, that bioptic drivers are more likely to be in an accident. It's a risk that, even if I had enough vision, I'd never take.. but you can't say something as blanket as 'they shouldn't do it cos they'll kill someone'.. because that's along the lines of saying 'deaf drivers can't hear horns, so they might not hear a warning and hit a chil running into the street, so they shouldn't drive!' ... ridiculous.
