I can't see why. It used to be that cameras didn't have built-in spot metering from what I understand, just center-weight and average. And heck, a lot of Canon models still don't have spot metering. But on a body with bulit-in spot metering I can't really see any reason to use a standalone one. (Flash and incident metering can be useful though).
Granted some of the standalone spot meters might have a few extra features like 1% coverage versus the 2% in Nikon's latest bodies. But the downside is that you may need to calibrate it with your camera, and you also aren't getting "through the lens" metering which means you have to worry about things like filter factors when using CPL's, light loss when using extension tubes, etc.
Spot meters are great for people shots and I use mine (in camera) all the time. For landscapes I generally use matrix metering; however, a spot meter can be used when you have a wide variance in light. Sometimes you have to decide whether you are going to let the highlights blow out or else lose detail in the shadows. With a spot meter you can make that a conscious decision vs. let the camera do it for your with matrix metering. Of course you have to know how many stops of light a camera can reproduce.
If you have time to wade through an extremely long (and at times technically intricate) thread, there is a discussion on the Adobe Camera Raw Forum in which you can read lots and lots of views on differences of metering between film and digital:
I typically use center-weighted metering in camera for almost everything, and sometimes double check things with the Spot, also in camera. Matrix is great but there's no real feedback to let me know the logic (or calculations) its using to arrive at the solution. Sure there is the histogram and the LCD preview, but if you know how to use a Spot you can still save time and get exactly what you want every time. I do often bracket with my Spot and Center Weighted shots, but I'll know wich directions to go in for any compromises.
For high contrast images, I would never use Matrix. Then again, if you can learn to guess at how your matrix meter will expose it, whatever is the most efficent workflow.