Everybody says this. Everybody who works at a web-based startup, that is. But I don't buy it.
For companies with more than 50 or 100 staff, the cost of in-house development is competitive with the per-seat cost of tools like salesforce.com. And you have full control of customization. And if something breaks, you have staff to fix it. Go with the big web based app, and have a problem, and you are less than nothing to their revenue stream, so you just wait and wait and wait for them to fix your issue.
Now, maybe in the bright future of personal hovercars and cities in the clouds (that'd be Web 2.0) these companies will bring the per-seat pricing down to a level where in-house development can't compete. But right now, they're priced as an alternative, not as an obvious, how-could-you-choose-otherwise decision.
Same thing with the "the network is the computer" folks. When a computer and monitor is only a couple hundred bucks, and good apps are free, why would I want to pay $20/mo for slow net-based apps and still have to buy the computer to access them. Turns out I don't do a whole bunch of weather forecasting or graphics rendering, so what would I do with 10,000 processors?
Why rent the cow, when you already own it? Jeez.