i think the reason consultancies (legal firms, accounting firms, etc) do it, and not traditional software companies, is because the worker hours translate directly into billable hours. Partners are incentivized to bring in more business, which the associates (and the partner him/herself) can bill against. I think partners are usually expected to bring in a certain amount of business to justify their profit share.

In a traditional software company, like a SaaS business, I don't know how it would translate. A developer fixing bugs might be important for the business, but ultimately is hard to translate to the bottom line. It's not like hiring a high level executive will have a guaranteed ROI in the same way.

Deloitte make a huge point of hiring apprentices - taking on dozens (maybe nearly ~100) of them on every year. Seems the general goal of everyone there is to make partner one day.