Couple of things that would prevent me from doing that.
1. I like to keep all of my music on my internal drive. Having the laptop in my office is convenient when prepping for a wedding and adding music to the database.
I have a dedicated laptop for DJing..music is on it. So what diff would a rack/tower computer make? None.
Yes, it would make a big difference to me if all my music was on a rack mounted unit with no real easy access without unloading the entire rack into my house. I also do lots of traveling for my day job, so I can take the gig laptop and prep in the hotel of an evening.
2. I'd be worried about travel vibrations. I'd feel more comfortable with my laptop secured in a cushioned environment during transport.
Maybe. My laptop travels in a padded bag, but then the speakers, amps, lights, etc dont' travel in anything padded.
To me, those items can take much more abuse and hold less value than a library of music. I can easily grab gear; not as easy to recreate a music library. But, my speakers and lighting do travel secured and padded.
3. I like that my laptop has the internal battery as a backup in case the power is tripped.
And so the laptop runs when the lights go out, teh speakers go off...
Yes, but with the laptop still running, it's a bit quicker to fire up everything else. And, suddenly powering off a computer can be harmful to it. Same goes if you're running an external hard drive and lose power.
4. Like Ron mentions ... more components and more real estate to be used with those components.