During various VDG discussions at Akademy, there was general agreement that the Translucency effect shouldn't be on by default. Entirely putting aside arguments based on aesthetics (which are subjective) and "datedness" (which are not usually appropriate since following UI fashions rarely bears fruit), there was a galvanizing event during this Akademy that revealed a convincing new argument: privacy.
Someone was giving a presentation during the KDE e.V. meeting and resized a Dolphin window to make it bigger. The act of doing this made the window translucent and revealed the content of the window beneath it. This was a significant privacy violation, and could have had damaging consequences. Consider that one could accidentally reveal trade secrets or expose oneself to personal embarrassment in the process of trying to avoid just those outcomes, because a window covering something sensitive reveals that sensitive content when moved or resized--even if the move or resize action was intended to make the window better conceal what was beneath it!
This effect does not seem to fit well with KDE's goal to focus on privacy. As such, I believe it makes sense to turn it off by default.