Transmitting a password in unencrypted form is not a good practice. It opens the user to hacking that could expose personal information and cause lots of issues (identity theft, etc.).
Sending BOTH the user id and password on the same email, without any encryption, is even worst. You are giving the potential attacker all the information in one spot.
Lots of people will receive such emails and keep them on their computer for reference. This means that anyone with access to their computer can also steal their login information.
There's a lot more to it:
The right approach is to double-verify the password during registration (enter the password twice) and then send a welcome email without the password. It is the user's responsibility to remember it. If they don't, they can always ask for a reset.