I guess in fairness I should point out the two primary purposes/responsibilities of Security Guards (as opposed to sworn law enforcement officers)...
-1- Deterrence -- by your presence, you should be a deterrent to criminals contemplating the property that you are guarding
-2- Observation & Recording -- if a crime is committed in your presence, you should be a good observer, and note all details of the criminals that could lead to their apprehension
(Yes, I'm paraphrasing from memory, and yes, I have a security guard certification).
But as for #1, I'm not so sure it would be a good deterrent to have a guard on a 5mph Segway guarding something valuable if I'm a criminal in a hoodie, and I can run 10mph for a long ways. Then again, most criminals can't run 10mph for very far...