The big reason, for me anyway, is Final Cut Pro. There's really nothing like Final Cut Pro on Linux (not the same quality, anyway) and Adobe Premiere for Windows isn't really the same thing.
Just use Toast. Easy as hell. Drag and drop the file, choose "no menus" or whatever the option says, and burn. Bam. Sure, you have to "pay" for this program, but it's still better and easier than any similar software on Windows. Plus, there's iDVD, DVD Studio Pro, etc. etc. Macs are ideal for doing almost anything with video.
I tried Burn, and then tried looking for some easier solution. No love. I also need to author BluRays. I don't mind paying for software. I have Nero Suite which works great. Unfortunately, my PC does not.