I've worked with all of the big five and I can assure you there is little to no value that they bring to the table. It's all smoke and mirrors dressed up in suits and sprinkled with corpo-talk that makes no sense if you're a domain expert.

Also, deploying clueless but well dressed junior people and only sending the seasoned specialists to extinguish fires is not a reasonable model. I mean it is if your sole goal is to rip off your client with delivering minimum value.