...it's about advancing to the next tech level OR expanding to a new city OR building up a fleet OR increasing your espionage abilities or any number of other things.
...Because everything can be boiled down to money, you have a real tradeoff - you have lots of other things to use money for. Buying more expensive rifes for your troops, or better missiles for your battleships, means you have less money to use everywhere else...[But] Games generally do a mediocre-to-poor job of representing this opportunity cost.
These two observations about opportunity cost make a convincing argument against the use of Research Points or any other separate and exclusive technology currency.
Games like Civilization V and Space Empires IV use Beakers/Research Points, which removes the opportunity cost. Normal resources that are spent on expanding a city, building a fleet, increasing espianoge, etc. cannot be used to purchase tech research. But in games like Starcraft and Command & Conquer the player spends currency that is used to build units and buildings in order to develop new technology.