Ideally, spoilers are to increase vehicle downforce or otherwise alter and improve a vehicles aerodynamics. In reality, 95% of the spoilers on current cars, whether factory or aftermarket, are purely decorative and worse than useless in that they actually degrade a cars aerodynamics for hollow fashion. There are a few that do actaully function aerodynamically -- Mitsu EVO and Subby STi and probably the Cobra R for example -- but the '05 Mustang's is a useless and rather cheesy looking deck ornament IMHO.
For my Stang, keep it mean, lean and clean, leave the jewelry for the Ricers and posers.
"It doesn't look like a Mustang without a spoiler."
Yeah, that's why I have such a hard time recognizing all those '64-'68 Stangs that never came with a spoiler -- they are Mustangs, right?