I'd think there would be so much coordination and onboarding overhead that it wouldn't be worthwhile unless it was so specialized/unique that your own staff doesn't understand it--and then why are you using software you don't understand and can't maintain.

Where I've had the most experience with using consultants and agencies for short-term/part-time are things like the following: -- You have a specific problem related to, say, ball bearing design and you really need to consult with an expert specialist. -- You need a speaker for an event and want a name of some sort -- You need a discrete project that you could probably do in-house but an agency specializes in that sort of thing -- You need a fractional share of some specialty (e.g. public relations)