1. It reflects poorly on my profession as a whole if practitioners are too lazy to fix or abandon obviously broken methodologies.

2. It's symptomatic of a general "good enough" mentality that makes software development much more difficult than it needs to be.

3. It breeds bad habits in novice developers that are likely to be my colleagues one day.

4. It steals mindshare and time that might much better be spent improving better alternatives.

5. It provides LAMP detractors with ammunition to shoot at the entire open source web stack.

6. I may one day have a half-assed PHP mess dumped in my lap to clean up.

Because it's bad for the industry as a whole. Clueless managers are looking exclusively for PHP developers because all they know is that there are a ton of them around. Meanwhile, people who've mastered more powerful tools and can produce more work of higher quality are being excluded from contracts for trivial reasons.

Because it's not good for the web if more newbies are using such a woefully insecure language.

Why is it not good for the web? Unless an insecure website is leaking personal information, I don't see how that affects you. Besides, how many websites designed by newbies reach hundreds of thousands of daily visitors (or even tens of thousands for that matter)? If said website is so terribly designed, no one would visit it because it would load slowly and be ridden with bugs; no fun.

HA! The LANGUAGE insecure? Yet another one who blaims the tool. Good luck with your career.

It doesn't help if the tool and its documentation encourages bad practices. I don't blame the tool really, I just don't think it helps.