I've been shooting B&W negatives for a long time, and recently decided to try to understand more about color.
As an experiment, I ordered a few rolls of most of the color neg film out there (at least in 120, which is my primary format), both for landscapes and portraits. Ultimately I found two truths, for me: first, I just didn't like the colors of any of them. They were okay, but just didn't move me as much as a great B&W film. The "neutral" films (Kodak Portra NC, Fuji 160C/400H/Reala) were boring, and the saturated versions of those films were, well, too saturated. (Fuji 800Z was okay, but still not quite right.)
Second, I had a bear of a time scanning! I know, neg film should be easier to scan, not harder, but every time I looked at the shadows, I just hated the grit in there. Not the same sort of thing as B&W grain, at all. Perhaps I need to expose color neg film more, but I used the same technique I've used for years with B&W, where my exposures tend to be just about right. (And I'm using an Imacon Photo, an excellent film scanner, so it's definitely not my gear.)
I should also point out that my ability to color-correct, well, sucks. When I see the image the scanner's made for me of the color neg, I can see that it's wrong (and often is), but I have a really difficult time figuring out how to "fix" the colors. I can see they're not right, but something about the workflow just doesn't sit well with me & my brain.
Recently I tried Fuji slide film, specifically Provia 400X. I totally love it! If I expose it correctly, the grain is aesthetically pleasing (or just not there at all), and the colors are just muted enough without being boring or bizarre. Scans were much easier and more right-on out of the scanner than the color negatives. (The Imacon has a great dynamic range, which probably makes it more suitable for transparencies than some lower-end scanners.)
So, my reason for using slide film isn't about projection at all, but about the "feel" of the colors, ease of scanning, and the more accurate rendition.
Granted, slide film isn't for everything. I still plan to shoot a lot of Neopan 400 in my rangefinder, and Acros and other slower-speed films on my Mamiya. I've got a Fuji S5 DSLR for low-light color and quick-turnaround jobs. But the Provia has a real place in my toolbox.