Why free software is important
The wide dissemination of and dependence on software by civilization supports a very important idea that Code is Law--it "dictates" from below; as such, it is important that software remains free (in the Greek sense of libre) to all those whose livelihood and future depend on it.
Societies have fought towards democracy (to elect those who rule from above), because until a perfect system is realized, totalitarian forms of governance can never be sustainable in perpetuity. Similarly, for this reason, software must evolve toward openness (which, by virtue of its reproducability lends itself to pervasive use and tends thereby to rule broadly or laterally). Copyleft sets itself apart from the destructive tendencies of proprietary, "totalitarian" software by preserving fair attribution, but requiring the software to be open and any modifications intended for public use to be offered likewise, that is, to share-alike.
Note that if a system of compensation were in place to fairly reward contributors to a project, it would encourage and feed continued community involvement and enhancement. The goal of metascore, et al., is very much like that--to put in place such a system whereby contributions can be scored fairly and contributors noted. Whence such a system is completed, the idea of "no commercial use" will become inapplicable as there will be no need to guard against the exploitation of creative value. Commercial use will come to be "community use".
Free and open source software is "viral" to inhibit the spawning of processes that would otherwise eventually tend towards dominance of the community-sphere. Without the "share and share-alike" clause, Will and secrecy dominate rather than the global community, OR (even worse) few participate because there are no guarantees that their efforts will be preserved for them (generally finding themselves back in a propriety system anyway. In this way free software acts as a sort of Article in the community "Bill of Rights".
Freedom is something that must be upheld lest all default back to the ground state of protectionism or, worse, be subject to willful exploitation. On this note, it is important not to confuse power with freedom. Although power grants a certain kind of freedom, in a global society it can only offer by itself, at best, a stand-off, failing to uphold true freedom. There will always be at least one other who will stand their ground to avoid exploitation. As such, the simple exercise and pursuit of power is a dead-end; much better is to see that both achieve more with cooperation and dialog. Building a system starting from free software ensures that the community built with it won't be locked into any particular solution to the question of how to organize it.
To appreciate the error in the common question "What's wrong with propietary software? Can't people just use something else if they don't like it?" one should look at world history and compare with "What's wrong with totalitarianism? Get out of the country if you don't like our system!". While one may have "freedom" in theory, in practice, such systems tend toward dependancies which are difficult to climb out of and, in any case, stifle creativity and innovation in the long run.
In short, Free and Open Source Software is "viral" in the same way that the U.S. Constitution is viral relative to each state within it. Copyleft's "share-alike" clause "reduces" one's freedom is in the same way that the U.S. Constitution reduces one's freedom. It is true only in a very particular way: to ensure continued freedom for all.