You really don't see any difference between guessing the value of a lawyer by unrelated extracurriculars and guessing the value of a developer by work on relevant open source projects?
Your question presumes that the extracurriculars are unrelated and that the open source projects are related. In that particular case I'd agree with you. However, I don't presume that the average programmer can anymore discern what open source projects are relevant, than the average lawyer can discern what extracurriculars are unrelated. I think in the particular case of law pro-bono cases and amicus curiae briefs would be more appropriate to juxtapose with open source contributions.
Similarly, depending on what field of law you want to go into and what types of people you want to work with as a lawyer you'd pick your pro-bono work with similar rigor to open source project selection if the end goal was career advancement.