Finally, some distributed version control systems, such as darcs, have no concept of revision number at all.

If there is "no concept of revision number", this is not strictly a version control system (IMHO).

Thereby demonstrating that your opinion is somewhat uninformed.

Note the original poster's comment was about distributed version control systems. In short they are version control systems whose purpose is to allow multiple source trees to grow in parallel while regularly passing patches between them. In this situation there is no central repository, and with no central repository there is no central notion of a revision number.

This in no way limits these systems from maintaining all of the essentials of a version control system. Such as an annotated history, the ability to reproduce the state of a file at any point, the ability to review each historical patch, the ability to tag a tree and the ability to generate a source tree that is the same as the version that was tagged.

The major strength of a revision control system is the ability to revert changes to a known point. This is why most will only allow a module to be checked out for update by one person.

If I had any doubts about how poorly-informed your opinion is, they would have just gotten resolved. Go off and learn what CVS showed the world about source control back in the 1980s. Once you've seen it work, well, I would outright refuse to work for a company that used a source control mechanism centered on locking access to files when people are working on it. Because if a company has that poor of a clue about effective source control, what other basic ideas are they missing? (Plus I don't appreciate my employer wasting my time like that.)