Naturally, it does subject the metal to direct contact with the elements; however exposed, polished metal is less subjective to rusting than unpolished, exposed metal. This is due to the more uniform and smooth surface than natural surface. I suppose it all comes down to your climate and intended use. Coastal weather would raise a caution flag.
I find it interesting that when oil is applied to polished metal surfaces, it beads up like water a rainx window unlike natural metal, it doesn't spread out, thus making it more difficult to oil a polished surface. I have not experienced rust of any kind on my Kel-Tec P3AT I polished (my polish test bed), and it's my daily pocket BUG.
In my unprofessional opinion, a polished weapon is worth the extra maintenance attention by nature, but to-date she's come up perfect. No rust here. I believe I speak for all the polished XDs here. To my knowledge there hasn't been any polish enduced rusting in this forum. But of course, I'm overly cautious with my handguns.
On the flipside, normal handgun use will create wear-marks thus exposing the metal anyway, so in conclusion I believe it's worth a polish job if you've already got some wear going. Anyhow, I'm rambling. I guess it's up to you. I've got an idea, how about a polish vs rust poll!