Integrating PDF support at the OS level makes it available to any application, and easy for developers to implement. It's built into the Apple print driver architecture. I can create a PDF of literally anything I can print, as easily and a lot faster than printing.
That means I can send anything I do to anyone, viewable on just about any desktop machine and most PDAs. It's also amazingly handy for relatively simple print jobs -- if I need to print something larger than any of my printers can handle, I just print to PDF and take it to Kinko's on a flash drive. I don't have to go through all the preflighting, don't have to worry about gathering up the supporting files.
It's a much more elegant approach than making the user hunt around for freeware apps and wait for a version of Open Office that might offer PDF support someday if a programmer feels like implementing it. It's not a critical feature, but it's very very nice.
Its still early, but I'm beginning to believe that MS botched the introduction of Vista more than the system itself by trying to convince people it would run on the last generation of hardware. Remember the "Vista Ready" and "Vista Compatible" stickers which have led to some class action lawsuits?
Apple, on the other hand, released an OS revision that runs, and runs well, on the last few generations of hardware. Imagine that.
Your points about printing to pdf are well taken. I use that quite a bit on Linux. I also print to postscript a lot, because I have tools that will let me do a lot of things to it later like reorder the pages for booklets and such. I really like the portability of PDF files, the only drawback being that unless you have Acrobat, you can’t edit them.