This has been brought up before:

scratchlive.net

(It gets discussed about 2/3 down the thread).

I agree, though, I'd like to have a single deck that plays through the built-in sound card for library management without the breakout box attached.

It could even have some sort of periodic audible tone or white noise (like many DAW plug-ins use for demo versions) as long as I could use it to set cue points. The typical form I see this protection method take is a 1/2 second burst of noise every 15-30 seconds. It would make it useless for mixing, but perfectly fine for track management.

To avoid driving people crazy, I'd have the protection tone only appear if the deck was enabled. If no deck was enabled, it would work just like it does now.

I was planning to start using Serato, but I'm concernedv about the mandatory use of the interface. It's Serato the only DJ software that requires an interface? The one that I've been using does not. Why is the interface needed?

...actually, let me take that back. The one I use has an audio interface, but its already built in. Perhaps Serato can make this adjustment as well.