I think swearing does make dialogue more natural, but only when used when necessary. Usually swearing indicates exasperation or panic, and those are good times to use them. For example if the captain is chilling out all calm and relaxed and a phaser beam flies over his head, he'd probably yell "Shit!". Bit more natural than "Oh dear, we appear to be in quite the pickle, number one." And since it's been brought up, we all know how silly made-up space swear words are. BSG only works because we've been conditioned by the show not to chuckle or smile every time someone says "Frak". I know I did for like the first half of season 1.
But if swear words are used too casually ("Engage the fucking warp drive and get us the fuck outta here, you fucking bitch. I got shit to do.") then it certainly gives the wrong impression of our heroes.
But if swear words are used too casually ("Engage the fucking warp drive and get us the fuck outta here, you fucking bitch. I got shit to do.") then it certainly gives the wrong impression of our heroes.