Jamie Zawinski: Unity of interface
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FTP, Gopher and HTTP are related in a way where it doesn't hurt too much if a HTTP client also support Gopher and FTP. However, there are cases where this doesn't work out so well (E.g. cross platform GUI frameworks).
Consequently, I'd say that it is nice to provide a unified interface for conceptually related technologies. On the other hand, I'd be really careful about overplaying this sort of design principle where it may lead to sub-optimal or overly complex solutions, simply for the sake of supporting more technologies (who needs Gopher these days?)