Low hanging fruits are a tenet of Open Source: work smarter rather than harder; reuse as much code as possible; release early and often. This way spare-time contributions bear fruits quickly. And yet there are situations when the fruits are not low hanging. Actually, the fruits do not exist yet. Somebody has to plant the initial seed and help a tree grow for it to bear fruits.
Seeding and growing takes time. A Google Summer of Code project is an opportunity for such a seed. Technically it is a 480 hours project all in one chunk, compared with the average spare-time contributor that puts in 2-4 hours per week and would take 2-4 years to clock that same quantity of hours (not considering the losses due to the pauses between the chunks of work).
So while in most cases the tenet of focusing the effort on the low hanging fruit still holds, it is important not to miss unique opportunities to add new seeds to a project.