The tightly-coded module written by the "expert" developer is a common occurence in software development. I could name several "experts" I would never hire again because of the overwrought, unmaintainable messes they left behind.
You make a great point: there is plenty of code out there that has comprehensive design documentation and is liberally commented, yet is completely inscrutable. Given the choice between a well-crafted system with little or no documentation/comments, and a poorly-crafted system with voluminous documentation/comments, the well-crafted system is a far better legacy.
Well-crafted means that the system has a clean design and that the code is clear, understandable, and easy to maintain. Cleverness is a liability. Simplicity is a virtue. While there may occasionally be a need for complex logic and code, it's not an excuse to let that happen to the rest of your system. I'll warrant that 98% of the average business system can be simply expressed.
Object-oriented development is partly about encapsulating complexity. If you can't eliminate the complexity, at least isolate and contain it.
The real solution is to not let it happen. Pairing is one potential solution: at least one other developer should fully understand any portion of the system. Or if you're not willing to pair, hold design and code reviews. If you let an overpriced consultant bang out a lot of code that no one looks at before he or she leaves, you're insane. If your consultant is too expensive or too arrogant to weather peer review, find another consultant to do the work in the first place.
Every shop has rules, whether they are written, oral, or inherent in the system. I'm not talking just coding standards, but process standards. If your shop insists on comprehensive
unit test coverage, the consultant must produce tests also. Make sure that your pricey consultants follow the rules everyone else does.
It may seem expensive to get a clever consultant to dumb themselves down so we can understand their work. But it can be far more expensive (and sometimes impossible) to smarten you and your team up after the clever guy leaves.
-Jeff-