I view the PICO III as a really well thought out instrument having everything you need, not just to get started, but as a full system. On the other hand I think the most active users of modular forums are more interested in making a highly personal instrument of all their favorite sounds, modulation and effects.
Both approaches are valid.
If you want to get started right away making bleeps, bloops and music the PICO III system (or any other full system or semi-modular) will get you there. If you're more interested in the finicky details of Maths VS Rampage VS Falisri VS Contour VS (insert dozens of others here), then explore by buying a module or two at a time and figuring out how they work together. Then adding or selling off modules as you go.