I have to disagree with you here. Just because someone writes poor code doesn't necessarily mean that they have the business sense of a rock. If someone writes crappy code but can market it better than the other guys, they'll probably win. I think Tumblr and WordPress both excel at blogging depending on the end user's needs. While they both essentially do the same things, each has its own merits. People who are looking to get into blogging or networking of any sort should definitely consider both. If they've only heard of one or the other and decide to go with it without researching, whatever they choose will probably work best for them.

Having waded through WordPress's code base for a few years, I agree with you to some extent. However, most people aren't going to care. Do you think half of the people who use MySpace, Tumblr, Twitter, etc care what it's written in or what it looks like inside? Facebook and Yahoo even use PHP and seem to do alright (though I admit they've modified it and have money to throw at problems to get them fixed). While I'm sure WordPress's code base drives some people away, people who don't know any better simply won't care.