Two reasons: (1) They pay their contractors well above market rates--usually a good 40-50% more than rates offered by other consulting groups or companies hiring contractors. (2) Complete flexibility. Project are staffed largely on a contract basis and you can indicate easily when, how, and to what extent you are available. Once you're on their list of trusted guys, they always come back to you--sometime too often.
Once you're on the list of trusted guys, they come back to you a lot--and, not that there's any overt pressure to work if you can't, but they often encourage you to try to make yourself available "as a favor." That's a good thing in that you're in demand and they trust you--but you do feel some soft pressure from time to time, especially if you're available 50-75% of the time so they get used to using you. Also, since most of the staff is contracted, there are no benefits. You get above-market cash based on hours worked but that's it.
Advice to Management
It might be time to ramp up your full-time staffing.
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