There's still this inexplicable attitude that DJing off a laptop is somehow cheating, inauthentic, and/or not "real DJing."
Many of the cranks spitting out these complaints are just woefully uninformed about what exactly is happening behind the computer screen - since they don't understand what's going on, they worry that they might be the victim of some kind of trick, and that the DJ might just be checking their email while a premixed set plays. Others are clinging to some classical vision of what a DJ should be, based on whatever era they came of age in, and see any diversion from that formula as a bastardization of the form.
Some argue that laptops just don't sound as good as vinyl, or that they're not stable enough to depend on ("I'll never have to worry about my needles getting infected with a computer virus"). Others blame digital music for the demise of the record industry, or assume that anyone with a laptop onstage is playing pirated music.
Myself, I've yet to adopt the laptop for my own DJ gigs - in the decade that I've been playing music for people to dance to, I've mainly relied on good old fashioned vinyl, and in more recent years augmented my usual crate with a bag of CDs. However, this is only due to my lack of a decent laptop, not because I've fooled myself into thinking that I'm somehow more of a "real" DJ because of it.
The fact of the matter is, those who dismiss laptop DJing are doing so because they're either uninformed about the technology or because they're uninformed about the history of DJing.
The tools that DJs use have changed many times since the early days in the 60s. If you came of age in the 90s, your vision of what a DJ setup should look like is probably along the lines of a couple Technics 1200 turntables and a Pioneer mixer.
However, if you were in NYC during the mid-seventies, you would likely scorn this version and insist that "real" DJs only use a stripped down rotary mixer like the Urei 1620 and belt drive turntables like the Thorens.
The first example allows for hip-hop style scratching, complicated long mixes using the channel EQ, drastic sound manipulation using the built in effects unit on the mixer, and rock-solid beat matching thanks to the powerful direct drive motor on the turntables. The second boasts much richer sound quality, but the lack of features and toys doesn't allow the user to perform many tricks beyond overlapping the tracks and fading them in and out, perhaps tweaking the master EQ once and a while for effect. Both are valid approaches, and neither should really be looked at as more authentic than the other - logically we should approach the technology of the 00s the same way.
Every previous era of DJs have eagerly grabbed up new technology and found interesting ways of abusing it for the service of the dance floor. If that hadn't been the attitude in the 70s, 80s and 90s, we wouldn't have hip-hop, we wouldn't have house music, we wouldn't have techno, and we certainly wouldn't have anything resembling turntablism.
In terms of sound quality, there are some valid arguments against laptops. Serato is pretty much the standard interface used these days, which uses special vinyl and CDs embedded with timecode data to control the digital audio files on the laptop. This program has been highly successful in part because it's not essentially different than mixing records or CDs - with the exception that you can bring your entire collection with you instead of just one or two crates. On the downside, you're forced to use their hardware interface, which many audiophile types have complained doesn't sound particularly great. Compounding this is the fact that many DJs are playing low resolution MP3s, which do have serious sonic limitations.
However, in the real world of club DJing the line is a bit fuzzier.
Unfortunately, many bars don't have their turntables set up particularly well, which can lead to feedback, skipping, channels cutting out, and a myriad of other possible problems.
While some of these issues also hamper those rocking the laptops, generally they're able to get around them with a bit of quick thinking, whereas the traditional DJ doesn't have nearly as many options. Furthermore, the sound quality issues of laptop DJing are all solvable by the DJ themselves - there are other programs that allow for higher quality audio interfaces and that can play files that have higher resolution than even CD - it's the fault of the DJ, not the technology if they choose to play low res MP3s. And while vinyl still measures surprisingly well under ideal conditions with a good pressing, most contemporary dance records aren't mastered or pressed particularly well, which means that conditions are rarely ideal.
As far as reliability goes, I've seen very few laptop crashes at parties, and at least as many technical issues related to traditional DJing. A few weeks ago I was playing for the annual Promise skating party at Harbourfront during a blizzard. The blowing snow was covering the equipment, and immediately shorted out the turntables, rendering the big bag of disco records I'd brought unusable. I made do with the handful of CDs I had, but ice started building up in some of the moving parts, which made tight mixes a real challenge. I was understandably jealous of the DJ following me, who'd brought a laptop and an all-in-one mixing board and audio interface - this high tech setup worked perfectly in nightmare conditions, whereas my supposedly reliable setup was verging on useless.
On the final issue of the relation between digital music and the music industry, it would seem to me that many people have an unrealistic vision of how the economics of the dance music industry work. While it's true that in the earlier days it was possible for a single to sell in the tens of thousands, that's a drop in the bucket compared to pop music sales, and for quite some time sales in the area of a couple thousand have been viewed as successful.
Over the past 15 years, dance music singles have primarily been promotional tools for the producers who put them out - they allow them to get gigs in other countries, and they can lead to lucrative remix work.
The sales themselves haven't been a real money maker for the majority of artists for a long time, and the rising costs involved in pressing and distributing vinyl makes the profit margin even smaller. Selling downloads is still in it's infancy and the margins are lower than you'd hope, but artists like Toronto's Deadmau5 are actually selling digital files in the tens of thousands on websites like Beatport, which shows that if anything, digital files might save the dance music recording industry rather than kill it.
If you're still not convinced that laptops are as valid as vinyl, check out what hip-hop producer Just Blaze had to say on the subject recently: