Why would you want to triple your equity partnership, as DLA Piper is now on course to do for its non-US business? After all, it flies in the face of a 20-year trend that has seen commercial law firms make up armies of non-equity partners to drive up their profits.

But if you step back a little and reconsider lockstep (or the supposedly 'merit-driven' models that look suspiciously lockstep-like on closer inspection) then the calculation totally changes. With a remuneration model that allows you to pay your top people widely varying amounts, then the question becomes: why would you want to keep massed ranks of non-equity partners?

Highly leveraged law firms work well in rising markets because the interests of fixed-share and equity partners are strongly aligned. But move into a hard-growth market and you are left with high fixed costs, volatile profits and tensions between equity partners – whose pay may go down in tough years – and salaried partners. A high proportion of equity partners also acts like a cushion in hard times, as it makes your costs more flexible.

Add in that high leverage can often lead to a greater reliance on debt, which gets more expensive during downturns, and the reasons to lower your leverage or have more of your partners within the equity are pretty compelling (so much so that you could argue that widespread expansion of non-equity partners is a distortion thrown up by the restrictions of lockstep).

True, leverage in the top 50 has continued to creep up since the banking crisis of 2008, with the group this year averaging 5.6 fee earners for each equity partner, against 5.4 in 2007-08. But if you look at many larger firms – including the magic circle – leverage has generally fallen. All things being equal, a large law firm with rising leverage over the last three years is a sign that something's going wrong.

These considerations have been weighing on DLA Piper's mind. As one of the most expansive firms through the 1990s and 2000s, leverage once worked great. But these days, a serious international practice with nearly 10 fee earners for every equity partner is most definitely pushing its luck. (I suspect, in this case, defusing inter-partner tensions and spreading the firm's capital base are DLA Piper's primary motivations.)

Yet there are two causes for concern for any firm expanding its equity ranks. A low leverage model with highly flexible partner pay means a firm must closely manage performance, otherwise such systems can become a licence to fudge quality control.

Even more thorny is the issue of profits per equity partner (PEP), which remains the share price for major firms. There's always been some truth to the criticism that the benchmark encourages law firms to structure their businesses to look good on that particular measure, rather than on the basis of what works for the underlying business.

With DLA Piper's PEP set to fall under this proposed shake-up, it will face that awkward reality head on.