Skillwires are for the jack of all trades, without a doubt. Even overlooking their limit of 6 (as opposed to how skills go to 12 now) they can't get specializations and you can't use edge. They're good because you can collect a handful of them at a decent rating (4 is usually fine) for skills you may need, but don't want to invest Karma in.
One adventure has the PC's offered a Pilot Aircraft skillsoft to use with hang gliders they're required to use. Another example would be something like Medicine, or Hardware or Mechanic skills, things you don't need to use a lot or be especially good at to get the job done. For a Street Samurai, they would free up their Karma to be spent on improving their strongpoints, such as bringing weapons skills much higher or improving attributes, while they can devote a part of their money into widening their skill set.
In particular, Knowsofts and Linguasofts can be very, very useful. They're cheap, and there's way too many knowledge skills to learn most of them. It's also much faster to buy a Linguasoft to learn, say, Mandarin Chinese for your next job where you're working with/against the Triads.
For Karma-heavy characters like Mages and Technomancers, if you're willing to eat 1 point of Essence cost, you can get Used Rating 4 Skilljack and Skillwires are character gen. This allows you to have a tangible thing to spend nuyen on from then out beyond the few things your archetype normally uses and lets you dump all your karma into your role without needing to spend a few to round out your skill set.
Additionally, if you are a technomancer, a Skilljack (which keep in mind, is where all your skillsofts are running, know- lingua- or active-soft) is a great target for the Machine Sprite's Diagnostics power, as you are without a doubt "using" your skilljack whenever you're using a skillsoft it's running-- With the new skill rating acting as the limit for the teamwork test. With a Rating 4 Skilljack, you can easily have a Level 5 or 6 Machine Sprite get 4 hits to effectively double your 'ware's effectiveness. I like to flavor it as the sprite funneling extra knowledge and experience from the Resonance itself to fill in the gaps in your software.