though i won't deny that having 15 visas in my passport is an asset, i would like to state that the letter requirement isn't enforced all too strictly, especially during times where our countries have friendly relations, such as now.

also, the letters needn't be official. a friend just walked into the dc embassy last week and came a way with a 2-year f w/ 200 days per entry solely on the basis of a large stack of emails. he hasn't been to the PRC in 5 years.

ps - i believe you can can officially get a 6-month L without any supporting documentation. L and F are functionally equivalent as long as you don't boast about any hot business endeavors.