Q:
Wait, no good reasons? Everyone has the same password, and I keep hearing that passwd is broken. Without the ability to change passwords, my phone is going to become one of those spam slaves I keep hearing about (and that's got to use battery life... serious battery).
A:
Actually, you've simply been corrupted by NullRiver. Their old "BSD Subsystem" package was slowly falling apart over the last year, and they never had the guts to admit that and switch to Telesphoreo. The userspace environment installed with Cydia has now and always did have a fully working passwd. Changing your password is simple and safe. (You should do this, by the way.)