Role of positions' geographical diversity in (US) academic career perspectives
If you are just talking about the geographical role, rather than comparing things like great position at a world renowned foreign university versus a two-bit US institution, then it is true that it can be harder to get a US tenure-track position coming from foreign university.  The main reasons are (i) smaller schools are often less willing to fly in overseas applicants for interviews and (ii) (for positions with teaching experience expectations) it's advantageous for candidates to have experience teaching in a system comparable to the American one.  See also this recent answer from RoboKaren.  Another possible concern is that you get involved in some research niche which is popular, say only in Europe or Asia, but not the US, but this can also happen at US institutions.

That said, you shouldn't take these concerns too seriously if you find a position that you like, and these concerns don't play too much of a role for research universities.  However, consider the possibility that you may need to do another postdoc in the US/Canada to improve your chances of getting jobs at smaller schools where this may be an issue.

Addendum: The above was just in answer to the bold question, but not to the issue in the text about whether it is in some way beneficial to go abroad, say strictly for diversity reasons.  Here there seems to be negligible benefit, all other factors being equal, as the US already has a diverse amount of resesarch, and one can still learn from and even collaborate with foreign colleagues (at least in many fields) thanks to travel and modern technology.