I would say it depends on marginal cost of growing in other markets. If you need to localize the site/content and/or need offline presence (sales team, etc), it might not be such a good idea. If all you need to do is strike a couple of content partnerships or PR exercise in those regions and can be handled by one person - you must go for it... Growth of Orkut/Linkedin/Facebook in India is a good example

Monetization might be lower today, but things may (and will) change going forward...