A lot of people 'wash' that spent trub/yeast cake and divide it into 4-6 jars, so they can make 4-6 more batches without buying more yeast.
If that doesn't interest you...yeah, just pour the new (cooled) wort directly onto the old cake...and wait for a MASSIVE fermentation.
It's all about 'optimal pitching rates' for yeast. 1 vial or smack pack is NOT OPTIMAL PITCHING QUANTITY. it might be fine...or you might get a slow fermentation with off flavors.
Making starters gets you to optimal levels to pitch...especially if you're making a lager or real high gravity beer.
Primary: English Mild
On tap: Pale Ale, Lancelot's Wheat, English Brown Ale, Steam Beer, HoovNuts IPA
Bottled: MOAM, Braggot, Raspberry Melomel, Merlot, Apfelwein, Pyment, Sweet mead, Cabernet
Gal in 2009: 27, Gal in 2010: 34, Gal in 2011: 13, Gal in 2012: 10