What a Leica is able to deliver is much better than most clients could expect!
Only snobery could turn down some Leica shooting in the field of fashion photography. Assuming you know your trade and are a
competent photographer, of course.
Most clients have bad taste and can't recognize good photography from mediocre, so they ask for BIG, thinking that size can substitute their insecurity (some photographers think like that too!).
In the past, you could see some guys shooting 6x7, then they would commission a lab to copy it to 8x10 inches, and would impress the client with a 8x10 transparency... The poor client would not even notice that he was paying some more for a dupe, for a fake... (The client wouldn't know how to differentiate the type of emulsion...).
I think that I shoot almost any format that you can mention from 35mm (and "cheap" digital=Canon 30D) to 8x10 inches, and I can assure you that size doesn't necessarily means quality. The nature of the assignment, the style and the technique required make me choose the equipment for the task, not the other way round.
Afterall, how many of us can claim to be better photographers than Jeanloup Sieff, Ernst Haas, Peter Lindbergh, William Klein, etc, etc? Even Ansel Adams used 35mm occasionaly...
(Yes, yes, I remember we are talking about "comercial"!)
I am not saying that 35mm is better, I am saying that it depends.
Only you can know what you wish to achieve...
Cheers,
Rui
P.S.: I have earned most of my bucks with medium and large format color shots. What I consider to be some of the best images I made were often shot in 35mm b&w... Now, sometimes, I have to shoot digital...
Ach, life ain't easy...