More screen-time for the Rommies would have been detrimental to their scariness. They worked because they appeared like a meteor our of nowhere, without any explanation.
How were they scary in this episode? All they do is appear, make an alliance with Picard, break off said alliance 10 seconds later and then leave. That's not scary, that's just weird.
I think giving the Romulans more screen time would have made them more threatening, rather then just confusing.