Disability insurance is all about the definition of disability. Only a few people have a truly comprehensive disability policy through their employer. This is because the definition of disability for group policies usually requires that, when making a disability claim, you cannot work in your job "or any other job" (or something to that effect). As as a result, you usually must be completely and totally disabled on a permanent basis to collect long term disability from your employer.
http://www.bogleheads.org/wiki/Disability_insurance
Often, they don't kick in for 6 months and then because you're still able to sell pencils for 50 cents on the street corner they will not cover you.
The reason why individual policies are typically much more expensive is because their definition of disability is different. Many individual policies are "true own occupation" which means that if you're now a neurosurgeon they can't force you to work at a McDonald's drive through.