Practice scratching?
I think you mean practicing how to move you mouse back and fourth really quickly. All software turntables are bullshit man... to practice beatmixing, there's plenty of good programs out there. I don't use any of them however. Buy a deck, thats the only way to learn... then buy a mixer... then buy another deck.
...the only two piece's of software I'll put
"DJ FROGi' STAMP OF APPROVAL" are:
MixMeister Pro, for it's incredible beat mixing ability.
And Stanton's FinalScratch, because it's the best freakin idea ever. Spin shit and cut up MP3's on your PC with your decks. Played around with a mates system, savin up dosh for my own.