Required caption

In HTML 4 the caption for a table is optional. In DocBook 5 with the 
HTML table model it seems to be required. Is this true? If so, why?

-- 
Elliotte Rusty Harold  
XML in a Nutshell 3rd Edition Just Published!
http://www.cafeconleche.org/books/xian3/
http://www.amazon.com/exec/obidos/ISBN=0596007647/cafeaulaitA/ref=nosim