Lean vs. "All-in" pre-ferments?
Lately, for enriched breads I've been somewhat lazy by mixing in everything (sweeteners, oils, etc.) into my pre-ferments, reserving only some yeast and flour for the final mix. I've found this simplifies the process, and, more importantly, seems to improve the taste. If I had to sell it, I'd explain that the ingredients had time to incorporate overnight - there's more of an uniform taste with more subtle notes as opposed to something that tastes explicitly flavored.
Based on most (all?) recipes I come across, however, it seems like standard practice dictates that the pre-ferment is made from grain, water, yeast or starter, and salt. Sometimes milk is added, but otherwise the "extras" are added during the final mix. Does anyone know why this is? Is there some advantage to keeping the pre-ferment lean? Or is that just a scheduling preference?