Lol, well yes, a safety earth connection is a good move (note, placing it under a wheel in contact with the tarmac is a VERY poor earth to the point where its not worth bothering with unless its raining very hard!

) .
So, why does an earth give us such a level of protection.
Well, lets consider the mains setup to your house.....the neutral connection is nothing more than a remote earth (at the sub station). As a result, the live return is via 'earth' (the neutral) and is referenced to earth (the neutral). This also gives you an insight as to how a PME bond can work where the neutral earth is good enough to support your house earth (they connect the neutral to your earth block where the supply enters the building)
And hence we use the earth connection to apply a level of protection as it gives us the ability to earth say a metal box which houses the electrical kit....the earth connection does NOTHING more than this!. We do this because it means that the user accessable parts will be at the same potential to the 'earth' you are stood upon and hence (in theory), no current will pass through you!
This is proven by looking at appliances which do not utilise an earth connection, they are considered double insulted as theere is to leves of insulation between the potentialy dangerous supply voltage and the user accessable areas.
So, is an earth always required, no......if an appliance is double insulated then it wont need one (i.e. a modern drill, TV's etc), if it has an earth connection (and probably as a result has a metal case) , then one should be provided.
A generator with a metal frame, should be earthed (I note some small ones are fully enclosed in plastic).
So, its not a cut and dried answer in reality.....and the chances are that the earth spike you drive into the ground to support this will not be a great earth anyway.....but something may be better than nothing.
The saving grace for a generator is that the live outut is referenced to the neutral but the neutral is not earth connected.....whence the live is NOT referenced to the ground which you stand on.....its effectively an isolated supply and hence its 'more' safe than the house hold mains.