When Charles Handy first talked about portfolio careers it was about something beyond the scope of traditional self employment - something more dynamic - fulfilling all the different needs of the worker and utilising their skills to the max. So doing one day of (the same type of) work for five different people each week isn't really a portfolio career.
My first 'portfolio' was changing from full time job to part-time (in the same role) and adding in a non-exec role in a different sector, plus some free-lance lecturing. So something stable generating a base income, supplemented by non-exec fee, and by variable income from lecturing.
Now I am self employed with a couple of regular clients (6-8 days per month), a variable and seasonal role in an entirely different field, and a long-term writing project.
For me the great thing about a portfolio has been the flexibility - each part can change and grow and develop (or stop) at its own pace. I can follow my instincts and dreams, but without risking my whole income - I'm a bit like a 3-legged caterpillar, with 2 legs always stable on the ground 
It also meant that when the recession hit, it only really affected one portion of my portfolio, rather than wrecking my whole world.
I love it, but it used to drive my Mum and Dad mad - they never knew what I'd be doing next and had no idea what to tell people when they asked what I did 