One reason to do it is cell count. There is probably 10x the number of cells in a half-quart slurry than the 11gram packet. I know it's not all that cost effective as a reason, but it's still $3.50-4.00 (two packs) in a 10 gallon batch.
It's always safer to pitch on the prestinely protected cake of a batch you just racked out though. The act of harvesting, washing and storing does give you plenty of opportunities for contamination. It's not something I'd recommend for novice brewers. Once you get through say 10 clean batches, no dumpers, you might feel like your concept and process of sanitation is sound.