I don't really roleplay when playing but at the same time, I love switching between outfits. I have my 'base' outfit as the mirror universe skimpy dress (main toon is female and HOT), partly because it's the only way to get the mirror universe skimpy dress by choosing it at character creation and partly because the character backstory is that she's from the mirror universe. Her hair is all ruffled 'cause it's a casual, layin' around, kickin' back type of style for running around ESD in.
Then for the 'off-duty' slot, I've actually renamed it "Combat" because I put her in the vest with the buckles and made it all black with tight leather pants (black) and combat-style black boots. Her hair is out of her eyes and pulled back, ready for combat. She looks ready to kick ass in that outfit. She stands out from her ground crew in it and looks all combat-leadery.
And then there's the fleet uniform which I use as a formal Starfleet dress uniform. Her hair in this one is slicked back and formal. I use that for when I have to meet an Admiral or Ambassador, or attend some sort of official function.
I know, it's kinda lame, but it adds just that little bit of fun to the game that I'd miss if I didn't have it.
What I really hate is that different slots limit your choices. All slots should have all choices. Anything else is just stupid.