Why wouldn't the public choose, e.g., GrassRoots, Google, or Microsoft Live for public computing services?

We don't discourage that. Those are viable alternatives that could be more cost-effective. In our experience, however, many organizations are hesitant about proprietary and/or ad-based providers.

Google Apps provide similar tools and similar rates for nonprofit web hosting. GrassRoots provides free nonprofit web hosting. Neither is specifically designed for charities to distribute public computing services, which might prove less cost-effective in the long run. Google is, of course, supported by advertisements, which could impact performance (negligibly, in our opinion), but might also include unpredictable content that could be contrary to an organization's core values.

To the best of our knowledge, each of these alternatives is built on proprietary software. That limits an client's ability to completely customize computing services (negligibly, in our opinion). In addition, in the unlikely event where one of those providers goes out of business, gets purchased by another provider, or stops providing those tools for any reason, their clients could lose the ability to provide computing services.

All of the services PCS distributes are Free and Open Source Software. Among other things, that means no one has to buy a license to use it. It also means no one needs to worry about losing it if PCS ever dissolves or evolves. Every line of code we create is 'donated' back to the open source community we derived it from.