I've heard some people complain about how the Cardassians were just lamer versions of the Romulans. Would you have been satisfied if all the Cardassian episodes were Romulan episodes and the Cardassians never existed (this spreads to DS9)?
Cardassians "lamer versions of Romualns"?!
Even if we were to accept that they are similar, it can only be the other way round, since the Cardassians, the characters and the society, have been a lot more interesting,well-developed and multi-dimensional than Romulans.
Romulans should have been Vulcans without the rule of logic and stoicism. They should have been as intense, volatile, powerful and dangerous as I imagine Vulcans would be if they were not suppressing their emotions and violent tendencies. But instead, post-TOS Romulans, I'm afraid, have been rather lame and disappointing, for the most part. Why don't they even have the same mental powers - telepathy, mind-meld - that Vulcans do? And think of how much better it would've been if they did not get those stupid, unexplained forehead ridges, which make no sense, because they're supposed to look identical to Vulcans, for logic's sake!
Think of all the possibilities if they have kept the ide about Vulcans and Romulans being genetically identical...they could have even had interesting storylines about Romulan spies posing as Vulcans.
And all in all, instead of being powerful antagonists, they have too often been just a minor annoyance or a negative reference point, related to their perceived inclination to be sly, lying and treacherous (Klingons and Cardassians consider "acting like a Romulan" as an insult).