The drumbeat of acquisitions underscores both how precious private-market data has become, and how limited the information still is. Pensions, university endowments and insurers now keep a lot of their money in long-term, hard-to-trade investments. But private-asset management firms often keep basic insights about those assets under wraps. Information their clients take for granted in public markets, including what investments perform best or the makeup of common indexes, can be guarded as closely as state secrets.