Look up some of the stories of people literally becoming homeless due to losing their coins when they were stolen from Silk Road 2. People aren't rational. They don't store "just what they can afford to lose and no more" in a webwallet, apparently.
You could counter that by saying "Oh, those are just drug addicts. Normal people aren't that dumb." To that, I can point at the example of the Mt. Gox collapse. There you have a case of people going from millionaire status to "looks like I'll have to find a job and keep it for a long time" status in the blink of an eye. One of the bigger losses was 4,700 BTC: http://www.reddit.com/r/Bitcoin/comments/1yv26o/gox_horror_s...
And lastly, you can't have it both ways. Either Coinbase is trustworthy enough to keep your personal savings safe, or it's insane to trust them with any amount of money that you care about not-vanishing.
You could counter that by saying "Oh, those are just drug addicts. Normal people aren't that dumb." To that, I can point at the example of the Mt. Gox collapse. There you have a case of people going from millionaire status to "looks like I'll have to find a job and keep it for a long time" status in the blink of an eye. One of the bigger losses was 4,700 BTC: http://www.reddit.com/r/Bitcoin/comments/1yv26o/gox_horror_s...
And lastly, you can't have it both ways. Either Coinbase is trustworthy enough to keep your personal savings safe, or it's insane to trust them with any amount of money that you care about not-vanishing.