Cons:
Since you make the tracks yourself, you deprive yourself from the surprise musical input of the other musicians who would be playing those parts
If the dance floor is full, and it would be a good idea to extend the song, you can't do that.
Notes Norton hit the big one right on the Head
The input of other musicians in a live setting definitely goes missing playing with tracks. I lamented it for awhile, now I pursue that feeling in other projects that are 100% live musicians.
As far as extending/contracting a song on stage, that's no longer a problem.
I perform with 320kbps mp3 tracks stored on ios devices. With the playback software I use I can easily extend/truncate/loop portions of a song.
At the gigs I use it to extend choruses of improvisation.
At practice, it allows me to loop sections that need more work. I can adjust tempo if need be to make sure a section is exactly how I want it.
This "con" can be solved pretty easily.