The first frame I built has a top tube drop of 1cm from the head tube to the seat tube. I did it to get an extra length in the head tube and keep a certain amount of seat post exposed. On a touring frame I dropped the BB 1.5cm to get extra stand over clearance and have less seat post exposed than the first bike. Simply for the look I wanted.
Yes, a sloped TT works just fine and can serve a purpose if desired.