Why? I've had a number of minor biking accidents that ended in a few scratches or grazes to me, and moderate damage to the bike - things like snapping both brake lever mounts, buckling the front wheel, etc. Even having to fix those things is a matter of maybe £50-100 tops, but if I had a brand new iPhone $whatever trashed as well, that would suck a whole lot more.

I think the large majority of serious accidents (to normal cyclists, not competitive BMX/downhill/stunts/etc) tend involve motor vehicles.

Unless you're hurt, alone, and in need of a ride home or medical attention. Been there, done that. It's a crappy limp home.

When mountain biking, you sometimes have minor crashes. I've had a couple where a smartphone mounted to the bars with something like this would be toast. I generally use some sort of garmin GPS, they are a bit beefier than my iPhone, also much better battery life for long rides.