EPTThere are two main reasons for this: One, this gives great insurance against economic harass because it doubles the rate at which the player can replenish their worker count. Two, it gives another source of Chronoboost, or in the case of Terran, extra MULEs and Scans.
The cost is fairly obvious - 400 minerals is a lot to throw out for a 'just in case' situation (losing workers to harass). However, increased worker production is useful throughout the game, and MULEs would pay off the cost quite quickly. Chronoboost is not as easy to quantify into a mineral value, but having a 100% increase in the number of Chronoboosts available (or a 50% increase if you're on two bases) should help a lot with upgrades, or getting valuable units (colossi, emergency observers) out faster.
I came across this idea a few weeks back, when I realized that despite the importance of saturating their bases, Protoss and Terran players are restricted by the speed of their Nexus or Command Center to produce workers. If a Hellion runby takes out six workers, you can't quickly replenish them the way Zerg can. I suppose the argument is slightly weaker for Protoss than for Terran, who gets a great benefit from a Command Center even without SCV production. Regardless, I have not seen this used by any player, ever, and I'm not entirely sure why. Is 400 minerals really so much that it isn't worth a faster saturation of your third base, or replacement after harass, or another source of Chronoboost?
Unfortunately I do not have any replays of success using this particular technique; it is pure theorycraft at this point. If anyone has a replay of a player using this, I'll edit it into the OP. Otherwise, discuss! Lambast the idea, debate its utility, or whatever you feel is appropriate. For any number-crunching needs, below is a link for mining speed references.
http://www.teamliquid.net/forum/viewmessage.php?topic_id=140055
