Martinz, perhaps the biggest problem is that 16 individual players, even if layered, do not at all have the same dynamic as a string section sound. In a section, not only do you have the minor differences between the players, which probably could be simulated, but more importantly, the interaction between the players, as each player adjusts to what the other is doing - not to mention more specifically following the directions of the section leader (In general, the section leader has the final say in determining the bowings and fingerings that the members of the section will follow). It is that dynamic interaction between the players as notes are played that individual layering cannot simulate.
Then, one gets into the problem of the number of MIDI tracks that would have to be created, worked with, etc. Both brass and woodwind sections are much smaller, and therefore easier to deal with than the far larger string sections. I would almost wonder if a Dimension Woodwinds would precede a Dimension Strings for that very reason.