The advantage is that it's more cost-effective, because you probably won't have to pay your superintendent or mechanic much more than you pay them to watch over 18 holes, and because some of the maintenance equipment sits idle a lot of the time so you can take care of 27 holes without 50% more equipment. Years ago, a friend who had overseen the expansion of a top course from 18 holes to 27 to 36 told me that the third nine cost them 25% more to maintain, and the fourth nine cost them NOTHING MORE, because they had gotten so much more efficient in their processes over the first few years.
The disadvantage of having 27 holes at a really good course is that people always want to identify what's the best 18-hole combination and play that, so they complain when the third nine is in play too often. To me, it's better to have the third nine designed as a contrast, instead of an interchangeable part ... but of course if the main 18 is undistinguished, then it's much easier to mix and match.