The most recent design to see fleet-wide deployment, the Mercury has already earned it a glowing reputation among the navy. While it's not as fast as the previous generation of shuttle, it has about twice as much armor and can take a hell of a beating and still get her crew home. Given how successful the deployment of the "Merc" has been over the last five years, the Bureau of Ships believes they've got a winner on their hands.
It is not designed to be fast, or carry large payloads or be a troop transport; instead, it's supposed to the jack of all trades. Reliability is key; the Mercury is designed with backups and fail safes so that it can take a beating in a combat situation and still make it back home safely. While it is lightly armed, having to use the weapons on a Mercury means you're already having a pretty bad day.
Designed primarily for personnel transport, non-combat missions, and light transport duty. While they can be used for combat in a pinch, they are ill-suited to it.
Basically, this is a Star Trek shuttle, no more, no less. Not even as cool as a Runabout.