You can enter a CP point and change units. If I'm going to be sniping and I can't rely on the bots or other players online to protect me, I'll change to a heavy trooper and lay mines in critical areas around me.. I'll then go back into the CP point and change back to a sniper. Sniper spots should be relatively close to ammo droids anyway don't forget. Snipers also aren't usually on the front lines fighting. A sniper is a camping unit.. and if you're going to be running around, you better be a damn good shot and a quick one.
Snipers are the lightest armored unit... so you move the fastest on foot and make jumps across things that other units can't but the trade off to this speed is that you really aren't protected when fired upon because you have hardly any armor... so you die in fewer shots than it takes other units to die from the same amount of gun fire.
If you're talking about single player in campaign mode.. you'll never do it with a sniper alone. Not on "Elite" anyway (I wouldn't know about "normal" mode because I've never played it
). SP campaign mode is designed to get you to use all the units and assets... including vehicles (perish the thought).