The "I" in BI should be about intelligence, not infrastructure. Most of current BI technology talk solely focuses on the "infrastructure" components - database, OLAP, workflow, data mining, reporting, etc. but just by having those components in one stack, you don't become intelligent. You still need data models optimized for your specific types of analyses, you still need to find the appropriate data mining and stat models for your problem space, you still need the right visualizations both to interactive analyze your data, as well as to publish your analyses in such a way that mere mortals can understand and use those insights to make business decisions. *This* is the real "I" in BI -- Intelligence, not Infrastructre!
Open source BI platforms are going to be key in highlighting this differentiation because open source by its very nature commoditizes the infrastructure of BI -- pushinig the proverbial "value up the stack" to components that actually produce intelligence.
At my work, we bet on this very phenomena where we utilize an open source BI infrastructure, and build domain-specific BI applications on top of it. Since we started doing this a few years ago, we didn't have the advantage to choose between Pentaho, JasperSoft, or BIRT -- so we did our best by building our own platform by integrating existing open source BI components, of which some like Mondrian and JPivot are also being utilized by Pentaho, et al. For us, this was a pure R&D initiative to leverage an open source development model, and in the spirit of collabortion, we even published it into sourceforge as OpenI (http://openi.sourceforge.net). We hope that as Pentaho, JasperSoft, etc. start releasing complete version of their platforms, we should find a way to collaborate with their stack. But for what it's worth -- you can download and use OpenI today as a BI application, just like we use it in production to serve our clients. In fact, I am keenly interested in community feedback on OpenI to understand the best ways to collaborate with the other open source BI projects.
The key is that open source BI needs to first define itself as a respectable, reliable alternative to the commercial BI vendors. Then we can bicker about what's free, and what's commercial. The infrastructure should be completely open, on top of which you can have commercial, domain-specific BI applications. But first, Pentaho, JasperSoft, BIRT, and OpenI -- all of us need to collaborate and put an open source BI stack out there that can hold its own in comparison to commercial BI vendors. Open source doesn't work like enterprise software where one company/organization needs to own the entire space. And even if they wanted to, they can't, because successful open source is all about building a community that spreads far beyond the confines of your organization. We are fortunate to have several noteworthy projects and components in the open source BI space -- it is time to these projects to collaborate and present open source BI as a unified front, and start growing the community. That's how open source BI is going to become real.