The biggest danger from a grinder is a burst wheel, which is potentially fatal, even with a small benchtop grinder.
There are other risks but grinding wheel which have a thin layer of abrasive bound onto a metal substrate are inherently much stronger and won't burst if they are cracked or damaged as a resin-bound grit wheel would.
A Howard states there are other reasons for having guards, but for metal backed wheels they don't need to be as robust.
Most importantly full guards should always be used when using resin bound wheels.
Neil