For the application of lifting a robot (or two) 12 inches, none of the sway problems stated above would really apply to the situation, and since you would only need a single joint, neither are the "every joint being critical" arguments. The single joint would allow you to Brace the lift in multiple places.(Shown by red circles in the below picture)
Now, in addition to keeping the scissor lift from swaying, the braces would also keep it from closing all the way, as is pictured below... but that is still adequate for the amount of space we have available to us.
A scissor lift could be a wonderful Idea for this years competition, It just depends which application you are using it for
For a scoring arm, however this would be an entirely different story...