Well, I think the author of that RFC was observing a pattern, not prescribing a rule. Rules should only be applied where they are in some way beneficial; and frankly I see no benefit to using three punctuation marks where one would suffice.

As Larry said once: "It's not really a rule; it's more of a trend."

UPDATE: Note also that the RFC shows (just below the bit that you quoted) that the "//" punctuation only appears before an "authority" (e.g. hostname). For example, the "file:" URI scheme doesn't use "//" for URIs of filenames that have no specified hostnames. Therefore, even the given RFC doesn't call for universal use of "//".

    -- Chip Salzenberg, Free-Floating Agent of Chaos