I am by no means a PHP expert, but I believe that its beneficial to be able to hack PHP files....I bought a few books and studied with an expert PHP and Javascript programmer, who understands the ins and outs of the DOM, but hasn't a clue as far as WP goes.
WP gives powerful tools and its not necessary to know PHP at all-- but it certainly helps when you have to customize widgets that do "almost" what you need.
Justin has given some great tools here, and especially for people who have had to dive into the PHP templates, understand Query_Posts and hack the loop. these are like WP on steroids....I can imagine why any developer wouldn't want to work with this theme.