That's a pretty easy statement to defend. But, I'll respond by saying that most Companies running Oracle 11g with more than a couple terabytes of databases, require a competent DBA, particularly if Disaster Recovery/Transaction Rollback is important.

" the DBA often never has sufficient domain knowledge of the problem "

Of the half dozen or so truly high level DBAs I've worked with (and managed on occasion), I can say they had incredible domain knowledge of Oracle Database Server, and worked extraordinarily hard to have next to zero knowledge of the application running on it. Their focus was to keep the database running, defend it from engineers and users, and recover it when things went really awry.

"DBs performance is complicated, yes, but the vast majority of it is extremely simple."

Any time you see the phrase, "Extremely Simple" when discussing a domain in which the expert practitioners routinely make $250K/year or more without any form of market manipulation, you need to reconsider why, exactly, these technicians are being paid so much to do something, "Extremely Simple."

"What you need to do is teach your developers how to read those query plans. "

Completely agree here, but, there are two perspectives on this topic. There is the "Engineers are ultimately responsible for the efficiency of their query plans, and should be educated/trained to take that responsibility" and then there is, "We can't train our engineers to be query plan experts, just keep them from shooting themselves in the foot, and let Query Optimizer handle the rests - it's up to the DBA to manage stats gathering to keep DBM_STATS healthy"

I think we tend to see the second approach more frequently in the enterprise, where your engineers are likely making less money, and the company is keen to leverages their many 10s of millions of dollars of Oracle Technology.

Finally, when a company is paying 10s of millions of dollars a years in Oracle licenses, they consider it a worthwhile investment to have a few high-level DBAs to fully leverage that investment.