I would suggest many climbing injuries and fatalities are cuased through that sort of decision (just ask Todd Skinner). However climbing is a dangerous activity and we all undertake and accept the risks we put ourselves in front of. A bike helmet is no more a better option than a fish bowl for going into space IMO (well, a slight exaggersation). As a concerned bystander, come see me between 9 and 5 at Rockface and I can arrange an additional 35% off discount on our already discounted BD Tracers thats a one time offer at just $65 for a quality soft shell climbing helmet. (offer goes to the first 5 people who see me personally and mention this thread)
If not, take care and safe climbing.