Working pro bono is quite a bit different than the cancerous scourge of spec / contest driven work.

Charities for example, might have a designer working pro bono. However, within that capacity, they are essentially working under the same constraints a designer on a team would. Many libre / open source projects don't have the funding nor structure to hire a designer.

Let it be clear, contests and speculative efforts are a toxic cesspool. Encouraging designers to integrate with a team, pro bono or otherwise, is a trend away from contests and speculative garbage.