I don't know too many actual pros personally.
I know a decent number and I used to correspond with one of Vogue's top photographers and while he has many assistants, he was very involved in every step of his photographs, including scanning them (this was the early 2000s), which is how I got to know him. I just wish I had kept in touch with him...
I naively thought most artists (pros?) were insanely protective of their craft, the process and most of all the finished product. Now I can definitely see a gifted photographer that cut teeth on film maybe having an assistant to help with all the tech stuff and working magic on Photoshop at the behest and direction of the artist but I would think most 'old pros' would feel silly having an assistant place their tripod and camera for them.
Its actually the complete opposite, all the pros I've assisted are more than glad to teach me everything they know. They know the real secret to success is not just your creativity but your connections. Which leads me to the answer for the topic.
I was one of those assistants that you're talking about for 5 semi-famous/popular fashion photographers in L.A. I proved to all of them how hard I want to work by being the best assistant I can be (it also helped that I have a bit of talent). They notice, they pass on jobs to me that they don't have time for or don't want to do. I got published in a fashion magazine within 3 months, I got free access to 3 studios/lights/equipment in 6 months (they trusted me enough to give me keys), within 8 months one of my shoots was big enough to be nationally televised ( http://www.junmapue.com/view/Rwv/Videos#cPx3
), by my 2nd year my 2nd mentor gave me a major client which let me shoot my first billboard, middle of my second year and I got hired to be Director of Photography for a couple of music videos (my first one debuted on BET). Currently on my 3rd year and I'm still surprised how I got here.
I rarely got paid for the assistant jobs I did but if I never did it then I would never have gotten the connections I have now. So now you see why us assistants do it. We know that's one of the fastest way to learn photography and get connections at the same time.