There's already plenty of ways you 'sneak' past someone if you want - any of the Invisibility spells, Hide in Shadows, Potion of Invisibilty, various magic items ...
Of course there's also various methods of seeing through those, whereas a rat would be the proverbial 'hidden in plain sight' - they can see you, they just wouldn't care.
In any case, I wouldn't call sneaking past someone cheese ... it's a perfectly legitimate strategy. If this was 'real' would you go stomping through the front door, raising a ruckus, and making sure the 'big boss' is warned of your coming? Or would you try to slip quietly by, and catch him/her by surprise?
On the other hand, in the game, you probably want to kill them for the loot anyway
