Why It Works
Desktop publishing and graphic design involve graphics-intensive tasks and Macs used to dominate in the speed at which they handled these tasks. Today's PCs have caught up and the speed differences are practically non-existent.
PCs dominate the corporate and consumer world. For those who need to maintain compatibility with those users, such as running the same office suites, email programs, proprietary software, and even games then having a PC will make that possible.
Older arguments in favor of the Mac center around its graphical interface as opposed to the text-based interface of the PC. However, Windows put a graphical face on the PC and with Windows XP the PC environment has caught up to OS X so that the Mac OS and Windows look and feel are quite similiar.
Why It Doesn't Work
For very high-end work where precision color-matching is absolutely required, the Mac may continue to have a slight edge.
In terms of cross-platform compatibility, Mac wins. Although there are programs that allow the PC to run Mac programs, at present, it generally works better the other way around.
The lower cost of PCs compared to Macs has been instrumental in the growth of the PC market. However, lower cost of the PC is generally only relevant for basic machines. To get the speed, storage capacity, and graphics capabilities necessary for most professional level design work will push the cost of the Windows PC closer to the cost of a Macintosh of comparable capabilities.
Some of the arguments against a PC are emotional or a matter of principle such as prejudice against Microsoft. Going with Apple and the Macintosh may be viewed as an act of rebellion against the norm of PC ownership. If it makes the user feel good, why not.
Where it Stands
As for working with commercial service providers, the growing number of PC files submitted by both professionals and non-professionals will eventually force more and more printers to adapt to the PC platform. The PC-based designer may have to shop around a bit more and possibly work just a bit harder to insure that the files they prepare are problem-free so that the Mac-based service providers will have less trouble working with them, but these are good things.
The old standby arguments for why only a Mac will do for serious design work are no longer valid. Either one will work.

