It hasn't got side skirts because it's not meant to, obviously. The S-Line sits inbetween the sport and S3 models so they have to draw a line with the styling. I think they've got the sporting balance just right.
I parked my S-Line bang next to a black S3 at the side of the road in sheffield on Tuesday night (anyone from on here??) and when you see them both together the styling of each model made a lot of sense. Neither looked 'better' than the other, just different. If you started putting S3 bumpers an skirts on an S-Line it becomes an S3 wannabe and i think it deserves more respect than that. The 2.0TFSI quattro is one of the only hot hatches about which doesn't look like one.....and i wouldn't want to change that.
By all means put some skirts on your S-Line, it'll look great but it's obvious to me why they don't do it as standard, it defeats the point.