Having experimented with the old gut setup, I think besides fingering ease, the old Simandl style fingering concepts maximized the tone quality of the gut strung bass. It is hard to get a gut A string going with the bow much past III postition because it is so thick - It just doesn't want to get moving. The tone of gut strings past the neck block is fairly unusable too. With steel strings, there are many more usable locations for notes so other fingering paterns become available.
As far as fingering difficulty goes, a bass with Pirastro orchestral strings and good orchestral string height still requires a lot of strength to finger firmly because the strings are so tense. Gut doesn't require that much more strength as one moves up the neck, it's just that the strings, being so high tend to go sharp and there is a physical time lag because of the greater distance to the fingerboard. Also, there are bow clearance problems in thumb position on the two middle strings.
That said, Nanny does explores four-finger technique in his book I, and even Simandl includes excercises that cover the lower strings in higher positions, so the need for these techniques has been known for a long time before the advent of steel strings even if they haven't been the first choice.
I don't think Zimmerman's fingerings are bad necessarily; There are just more convienent and possibly better sounding alternatives now. I've always felt that one should learn all the reasonable fingerings for excerpts and solos so they aren't limited by fingering habits and can instead make an educated decision based on the requirements of their own bodies, their equipment, the particular music and playing situation.
-Jon
