What allies? You mean the Klingons? Who said the Klingons never saw the Romulans, or are you talking about something else?I'm OK with the idea that the adversaries never saw each other ... but I thought it was idiotic that the allies never saw each other, either.
From Balance of Terror:
"...As you recall from your histories, this conflict was fought, by our standards today, with primitive atomic weapons and in primitive space vessels which allowed no quarter, no captives. Nor was there even ship-to-ship, visual communication; therefore, no human, Romulan, or ally has ever seen the other. Earth believes the Romulans to be warlike, cruel, treacherous... and only the Romulans know what they think of Earth. The treaty, sent by subspace radio, established this Neutral Zone, entry into which by either side would constitute an act of war. The treaty has been unbroken since that time...