The reason for all the attention is how quickly the meaning will change. To say "I have a sore head, somewhere" ("đầu của tôi là đau đâu!" in Vietnamese) is nearly impossible for (foreign) learners on their first day. A learner will only hear 'dow, dow dow' or 'dough, dough, dough' or 'dow, dough, dough' which does not help when listening. The same problem can be stated in English - 'four forks for foreigners' foreheads forthwith!' 