Warren Buffett will go to great lengths to stop Berkshire Hathaway using consultants. "If the board hires a compensation consultant after I'm gone, I will come back," he promised its annual shareholder meeting on Saturday. It was a joke, but from the heart.

Mr Buffett's low opinion of overpaid hedge fund managers, whom he describes as "compensated on the basis of something that in aggregate cannot be true", is well known. He extends that scepticism to other intermediaries - investment bankers, brokers, and consultants of every kind.

It makes him highly unusual, since the corporate world is heading in the opposite direction. Everywhere one looks, there are more and more consultants - on strategy, investment, operations, compensation, digital transformation, technology, marketing. Some businesses seem to have been entirely occupied by consultants.

What was a niche for a handful of wizards in firms such as McKinsey & Co and Boston Consulting Group has turned into a thriving industry that keeps on growing faster than many of its clients. Consultants now do many jobs that companies once performed themselves.

Global revenues of management consulting firms grew 7 per cent last year to $US133bn, according to Source Global Research. Some top firms, such as Bain & Co, are enjoying double-digit expansion, and have for several years. The large accounting firms, which mostly left consulting in the early 2000s, are back: 44 per cent of EY's revenues last year came from advisory work.

Consultancy is at the heart of professional services: of 2.2m people who work in financial and related professional services in the UK, 477,000 are consultants, compared with 421,000 bankers, according to the City UK trade group. What is going on?

One answer is that they are needed. Many companies shed employees and retreated to their core after the 2008 financial crisis, outsourcing activities from manufacturing to technology. Hiring expertise as required - management as a cloud service - is a natural next step.

Technological change is also a boon for consultants. The rise of digital technology and data analytics is upending many industries, including retailing and media. Companies wanting to remake how they operate have turned to consulting firms, particularly larger ones with tech expertise.

That can mean everything from how a company collects and analyses data to how it advertises products to consumers through Facebook and delivers them to their homes. Marketing companies such as WPP and Publicis compete with digital divisions of consulting firms such as Accenture.

These changes have serious implications for the nature of the company itself. A generation ago, the chief competitive advantage of US corporations such as General Electric and Procter & Gamble was management. They trained cadres of executives to run operating subsidiaries smoothly.

Consulting offers a substitute - the ability for companies to outsource chunks of strategy and operations. It begs the question of what a large consumer goods company, for example, does. If trends continue, it could soon amount to a few managers overseeing consultants and contractors.

Even for those that do not go that far, using consultants has attractions. It offers flexible use of a well-trained group of managers and professionals - the top echelon of what Accenture calls the "liquid workforce". Hiring consultants who have undertaken similar projects elsewhere is reliable and fast.

Off-the-shelf solutions

But there are dangers, which can be overlooked amid the rush. One is that companies are buying off-the-shelf solutions that make them operate more like others. They are being sold similar ideas and similar methods for reaching customers. The curse of the consultants is that anyone can hire them, so their ideas soon spread.

It is a version of the conformity problem in asset management so scorned by Mr Buffett - originality is very difficult to come by. Berkshire is deliberately eccentric and obstinately operating in its own way has produced enormous rewards. As he wrote in his 2014 shareholder letter, "there are worse things in life than having a prosperous business one understands well".

The second danger is that consultants become a habit - once they get inside the building, they are hard to eradicate. They have an interest in keeping the relationship going, either by persuading clients that the challenges are complex, or by selling them more services.

A company that needs a few tasks done quickly can become enmeshed. Many US corporations have "an expensive bureaucratic culture internally while a growing corps of financial advisers bids to sell shareholders a wide range of costly services", writes Lawrence Cunningham, a professor at George Washington University.

Consultants are alluring in a world of changing technology and regulation, as their growth shows. But it is also wise for the buyer to beware of what they offer. One thing we do know: if Berkshire ever joins the bandwagon, Mr Buffett will turn in his grave.

john.gapper@ft.com