I dunno, it was pretty obvious to me the first time I saw it. Short term gain/long-term loss (most of the time).
However, if it means keeping the city, or having a major unit early in the game (that lets you get access to an area before someone else), then it can still have long-term game ramifications.
I think it's a perfect option alongside the others. I tend to play the "improve my cities" approach, and so I pick one of the % options most of the time. But I have made a city more than once that was more strategically placed for bottlenecking instead of focusing, and picked the guardian option a couple times.
It was nice to have those free units to quickly clear out an area and gain a couple prime spots for gold and tech producing cities.
Getting something a number of turns early can mean a huge difference in a game like this. Think of it as an invisible long-term gain, that depends on how you use the resource.
