I'm glad I read this thread, as I have to admit that I probably would have continued riding with a crank like that until it broke. On reading the replies I have changed my mind, so if it ever happens to me I will be replacing the crank pronto!
Just to confirm what has been said already, never ride with a damaged crank , I've broken two , both Campagnolo, and it's no joke. Also check out handlebars especially around the extension clamp. When they snap there is generally no warning.
Keeping old bikes on the road may be a pleasant occupation but I've fallen off too often to take any unnecessary risks nowadays and keep old bikes just to look at now and then.