You need warm bodies on a site... period.
A site that only has 15 people using it is... sort of pointless and possibly not really worthy of graduation. Having some chunk of users with a level of minimal reputation means that there are actually people using it and they will become the voting base for elections (at 150 reputation).
As such, this stat seems largely pointless in relation to graduation.
It becomes even less so when you consider that, with the release of Design Independent Graduation, the reputation levels don't increase when the site graduates and these stats existed before this change.
The most notable reference point for 200 reputation is the Association Bonus. At 200 reputation, we trust the user enough to give them a 100 reputation bonus everywhere on the network. As such, despite there being no additional privilege granted at 200 reputation, it is an important rite of passage for a user.
Is this stat useful, though?
Earlier I note that this stat seems pointless... I would hope that you could argue that any site that maintains 10 questions per day (the primary goal for graduation) will surpass this goal easily and that, if it does not, there's a major failing of the active users of the site to actually vote on content. When a site graduates in the "target" of 90 days, those 200 reputation users are probably still pretty close to being active...
However, when a site takes 2-3 years or longer to graduate, the opposite is the case. Only the most active users are around and there are few users available to actually run the site and, when this happens, you don't have users willing to run in the site's first moderator election or to vote in it, and you rely on a core of very few users to actually keep the site afloat.
This is dangerous and is a real problem! As such, it might be worth considering how recently the reputation was earned when assessing the sites' ability to maintain themselves once graduated.
A site recently graduated. These were the stats:
So, despite having a very low answered percentage, low answers per question, and barely meeting the avid users stats (these numbers are after three years in beta)... this site was graduated. Upon starting their graduation election they had insufficient users interested in running in the election, and so it was cancelled.
Now the future of the site is somewhat unknown. This ... is not good. These stats are supposed to ensure that a site will be successful when it graduates... and yet, despite currently sitting at 20 questions per day, this site is in trouble because it doesn't have enough active users.
So, yes, this stat is important but I don't think it's being assessed correctly, since it's all-time reputation rather than something more like last 180 days.