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"Strange, distros generally put the "open" or "community" as the same level as their commercial version, minus some functionalities ... so that you have the same quality, but not the same functionalities, the diff is the price."
Not really, at least not in the enterprise realm that we're talking about here. Compare the latest RHEL and the latest Fedora, very different. What you're getting with the freebie version isn't just an identical product minus some plugins or whatever, it's a testing ground for largely bleeding edge beta software which is expected to eventually, in some iteration or other, find it's way into the enterprise line. The plus of the free version is, well, it's free. The plus of the enterprise line is that it's expected that the bugs and annoyances of the free version have been resolved, and that you can deploy it with the confidence of a "supported" product with a longer life cycle.
At least in the case of SLED/SLES and RHEL (which in the enterprise linux line are the only two real players at this point) this would appear to be the case. Minor distros without a strong enterprise presence of course can differ with regards to the pay for and free version, I think that's probably more what you're thinking of.