I'm a light medic because I need to be able to get to downed people fast and get out faster.
An excellent tactical idea.
As stated in my sig, I plan on running a Medium Operative for a few reasons:
1) I don't think I need to climb up and over a full story of stuff; once I learn the area I'll figure out creative ways to use SMART to get me where I need to go.
2) Having low health would irritate me: I'd like to be able to take some punishment in a straight fight
3) I'd like to be able to at least have the option to use heavier (meaning medium) weapons if I want to.
I'm fairly certain that with the right abilities and some amount of creativity I can make a Medium Operative (presumably a role suggested for Lights) a very successful character.