I run a main dealer parts dept and deal with obviously both our own technichians and those of independents.
It dosen't matter how good your fred in the shed is, they will be faced with a different brand of car every half hour or so and so probability dictates that he may have a harder time fixing your car should a problem arise.
In a main dealer, even if your car ends up on the least experienced technicians ramp his colleague next to him, and the one next to him also, will always be there to call on for help. This aswell as access to the technical support team from the manufacturer and the fact that the last car he worked on was probably similar to yours.
Not having a go at independents at all, just saying what I see everyday.