It can be a useful metric for some cases. It's very likely the price at midnight of some major timezone (PST, EST, GMT), but someone should try to confirm when it changes.
EDIT: It's GMT. The interactive chart shows 12:00 AM GMT as time of opening.
But this is actually aggregation not reality. If u look at BTC-E, BitStamp or LocalBitcoin, trading takes place round the clock. There is a big chance that non-bitcoiner financial people may follow Yahoo chart and that would result in wrong calculation. Opening & Closing does not apply to crypto trading !!!
I'll disagree. I don't see any material miscalculation that could come as a result of showing an open and closing price. If nothing else, it gives a nice way to summarize the price at the same point in time every day - i.e., if I wanted to see how much the price changed daily over the course of a year, using the open price for each day would be a nice alternative to a weighted average.
If you're really concerned about it, contact Yahoo and see what they say about changing it. I think that more information is a good thing, not a bad thing.