Way back in 1993 when I used to play SRII, I had an Elf Sammy girl who used to carry around (or wear, depending on the situation) a pair of roller-blades.
I must confess that I'm not at all familiar with the SR4 Arsenal supplement, so I can't comment on it nor how the authors present the item in question. That said:
Skates are light-weight, portable, legal, and fairly easy to get a hold of. When you need a little speed boost that's not on a grand scale, from something that you can carry around with little to no difficulty, they can give a fair advantage for a number of things:
-Jumping
Sure, it's a risk: but you can build up enough speed to plausibly increase the distance of a long jump (say, from rooftop to rooftop).
-Increased movement rate
Not sure how movement is handled in SR4 (I'm getting my copy soon-ishly), but it is very much reasonable to figure that wearing a pair of roller-skates would increase the distance a character can move during their action(s).
-Maneuverability
In a tight spot, and by inventive use of the surroundings, sometimes being able to skate instead of walking or running can give an advantage. Granted, in cramped quarters, the opposite would be true.
-Quick getaway
As has been mentioned earlier in the thread, the daredevil maneuver of gripping to the back of a speeding vehicle (although likely to be much, much more difficult to do than in movies) could plausibly help the character oust his/her person from a scene more expediently.
The most obvious downside is having to do with putting on the skates and removing them, as it's likely to be something that ought to take more than three seconds (or, at least, an action or two). Therefore, the choice a character must make is to either A) arrive at the scene with skates on (which can and should prove problematic for, say, stairs, or might attract attention, or any other situational circumstances that ought to apply) or B) find some out of the way spot where they can be put on in peace.
As for Skimmer Discs?
I'll venture a guess in that they may be more expensive, more difficult to carry around undetected (at least, maybe they don't fit as well into a simple backpack in comparison to roller-skates), not as easy to find and acquire as roller-skates are (unless Skimmer Discs are available at your corner sports store? Genuinely curious, am clueless on the matter).
Plus!
...
Them's fun.