I see U.S. coins as just another country of the world, so of course I collect them. But collect whatever you want, Its your collection.
I agree with rdwarrior. Most world coin collectors will collect US, since its a country. However, most US coins are way too expensive versus world coins, so they usually do not collect US much. I consider myself a world coin collector, its just that the majority of my world coins are simply older than most others. I consider ancients a world coin, simply a little older.
For collectors outside your country, the US is a country like any other, so an OFEC collector for example would include them of course. Christian