On the other hand, you may end up with what they call a Kigyo Shudan, a type of horizontally diversified business group found in Japan. Such a corporate structure could be beneficial if there is a lot of synergy between the participating companies, especially synergies where individual companies can increase their value by strengthening business and technological links with each other.
The Small Business Web is a group of companies that voluntary cooperate with each other for the greater good. Involved equity exchanges between companies should foster self-interested cooperation.
However, I do echo Cubes' sentiment that any equity exchanges should not diversify founder risk so much that they rest on their laurels or focus on the relationships with others to the detriment of their own startup.