The situations where it makes sense to bring in outside expertise are.... Sometimes public servants will say that, for example, for surge capacity. If there's a sudden need for talent and you can't in particular hire them or it might not make sense to build a permanent team, it can be smart to work with outside players. Sometimes there's a very specific expertise you don't have on hand, so leaning on outsiders for that should be encouraged. We don't want an insular public service that presumes to have a monopoly on knowledge.
Where it becomes more problematic is on bigger questions around the strategy of public management or how a program is designed and who it serves. Multi-year transformation strategies are another area where a lot of these firms get really involved. From what I've seen, I don't think they are producing much of substance, but those are the kinds of things that should be treated as core public service work.