Oh I completely agree with you. To me it just doesn't seem plausible. But when "Balance of Terror" was written, I'm sure the producers were thinking "Huh, a hundred before the series starts, I wonder what technology would be like then..." and based their predictions on what the technology and weaponry of the 1960's was looking like and how it might develop in the future, as well as comparing to the tech and weapons of the 1860's. That's always the tough part about sci-fi... trying to predict how and when certain technology would develop.
So to us, living in the 21st century (40+ years later), the idea that we would have an interstellar war without FTL drive and never, ever, ever seeing our advesary (and not just us -- the Vulcans and other allies as well)... That's just weird.
And yet, we're stuck with it because it's "canon." (Though I agree that there comes a point when "canon" is just silly... but some fans are quite vigilent.)
And if I were Spock, I'd totally do the "wait, what??" thing at seeing the Romulan commander.