Too bad as it's a great application.
I don't know where you got that idea, but try walking into any printer's office and saying that. They could use a good, long, hard laugh.
You're right that Pages, part of iWork ($79) is the best option (well, for people who want to produce good-looking professional stuff, that is). There's some excellent postcard templates built right in.
For more casual stuff, I like
Swift Publisher from BeLight (makers of a number of good graphics apps), or your suggestion of iStudio Publisher.
If you're strictly wanting to do postcards -- nothing but -- then yes,
PostCard (as referred to above) is an excellent choice, and you might want to check out
Posterino as well.