I agree. They just cheat themselves.
Elite runners do not cheat (scored whole trifecta and run in elite wave). At least I have not seen. They also disqualified if they cannot pass some obstacles.
There is always some volunteer watching by each obstacle but in some locations volunteers are not strict in penalising or just do not care much, like during last one I ran. In the one before, they watched and where well trained in penalising. But there are still some parts of the trail without obstacles where there is no-one watching so they can cheat running shortcut, or omitting water or hill which I have seen few times.
The trail varies according to location but it is seldom asphalt road. Mostly park or countryside trails. The Beast I ran a month ago was hell. It was mostly sky running trails with steep few hundred metre long approaches. You do not run then, just slowly push walking up.
I have not tried other trail trainers by other companies. Only Reebok. I have some normal running Asics shoes which are decent.
For SRace I have used some old Reebok trainers designed for this competition. The cheapest model that was in series with normal laces (but I bought those plastic, sliding ones you mentioned from ebay and replaced). They have decent grip in front and I ran 5 Spartan Races in them and will run 6th soon. I just carefully hand rinse them after each race (takes a while) so they are more less clean, then dry.
Some people throw away shoes after 1-3 races.
When preparing I use different Reebok model, I got few months ago. Decent front grip for traversing and climbing walls, and lighter than the other model I have got.
http://www.reebok.es/all-terrain-super-or/AR0053.html
Reebok usually makes 3 new models each year for SR with different prices, grip, weight and shape.
There are also other popular mud runs around if you want to try. I need to try myself too.