Here’s the rationale behind disabling the welcome email as quoted in bp-core-activation.php:
* Since the user now chooses their password, sending it over clear-text to an email address is no longer necessary. It’s also a terrible idea security wise.
I’m going to sway a little to Peter’s side and say that BP should leave this option to the site admins. I actually think there should be more options for the administrator on a fresh install.