I've found the best reason to build a monolith is you get production from all three resources from only one facility. While they may be expensive you gain a lot of extra space for other uses.
quote:Originally posted by Daynarr: ... when you build multiple value improvements on planet ... all resources (except those that have 0% value) will rise and make your investment pay off.
Even those that have a 0% value will raise. Remember, the increase isn't a percentage based on the current value of the planet, but rather percentage point(s) added to the value of your planet.