I agree with the more than beer money aspect.

I have a print I made in 1975 , it was for my third year portfolio and it hangs in my darkroom now. I had given it to my Dad back then and he passed away just last year so I have it back.
What struck me as odd is how I feel its every bit as good as the work I am doing now for myself and others.. I may print it slightly different now but its pretty good.
I think the difference of printing for others is it trains you to print in different styles, contrasts and density. I think you become much faster each year and more confident when you lay down tones.

I do not think after looking at this print that you become any better, just a bit more adaptable to other styles.