This is something that the guys at mt LHBS had to talk, and talk me into doing. Surprisingly so as it knocks off $6.50 that I would pay them for packs each brew.
Long and short, they say, "If the batch you rack out of it isn't infected, why would you assume that the stuff left behind would be?"
After racking out of Primary, leaving that 1/4-1/2" of liquid and the cake put the lid or stopper back on with the airlock. This keeps anything from getting in. Be ready with your next batch and dump it right on top. You can do this upto 6-7 times.
It makes since to do a similar style of beer or plan on darker brews each time.
It saves you money and cleaning time.
And the yeast you 'train' during this process gets more aggressive each time.
I pitched a 1.060 Brown Ale onto a yeast cake on Saturday and it started rocking my air lock within 20 minutes and needed a blow tube within 3 hours. Checked it this morning and am at 1.019 already. This will be a 4-5 day fermentation period. Just like the local Brew Pubs.