The first thing I do when I buy vinyl is to record it into my computer as mp3s. Not for "back-up purposes" so much as because it forms an easy list of what I own, and I can easily play with the music on iTunes to build playlists. You know - "what does this one sound like before that one? does the ending of this one go well with the start of that one?", without having to actually drag out the records and play them. And, as it ends up, I happen to own the album on CD.
But I don't do DJ gigs with CDs or mp3s - just vinyl. To my way of thinking, that justifies my presence at the event. If I do it from CDs or mp3s, I can just do everything at home, save it as one track, then show up and hit "play". In essence, I could just hand them a CD and not be there at all.
In essence, I've built my own little box, and I'm forced to work within it. (Most of the time, in fact, my guidelines are even more stringent - I only DJ using 7" 45rpm records. It's only recently, within one "format", that I use LPs and 12" singles.) If I use mp3s and CDs, literally everything ever recorded is at my fingertips. Hell, MORE than that - I can futz with songs and sounds and create new ones, and DJ with those. But "within my box", I'm a lot more limited. I have X number of songs, and I can't alter them much. (I can speed them up or slow them down, or start them late or end them early. That's about it.) Given that, can I create something worth listening to? I like the challenge of operating within those parameters.
Mind you, I'm not dismissing or criticize those that DO use CDs or laptops to DJ. Trust me, whenever I have to cram two turntables, a ton of records, and a sound system into my car, I completely understand the appeal of laptop DJing.

And a lot of them are brilliant at what they do. It's just not what I do.
Lex